First, feel blessed that you have parents who want to buy an extra 100 points and add you to the deed. As to some realistic benefits and disadvantages:
Perks: if they purchase from Disney, you will be entitled to all the same perks they are entitled to such as discounted APs, the 10% merchandise discount at WDW stores, others. If they purchase resale, that contract will be unusable to reserve the Disney Collections, and, because there will be a new member number attached to it, you will not be entitled to the perks under the rule change that was effective April 4; they will still have use of the perks because they already own other contracts.
Transfer Rules: you can transfer current use year points, but not points that have already been banked or borrowed. A member can bank points already transferred into his account but cannot borrow them. The use year and banking deadline for transferred points retain that of the transferor. A member (actually a membership number) can make only one transfer per use year, either in or out but not both. Thus, your parents and you will be able to transfer the points from the new membership number account to your parents' old membership number account only once per use year. That becomes a consideration when you believe there will be some points not used for a trip that will likely be banked. If you keep those extra points in your account and bank them, then those will not be transferable for the trip in the next use year that uses them. As a result, you may want to just transfer all points every use year. As mentioned, if your parents also create new memberships with other siblings, you will then have a transfer issue because the parents can receive in their original membership only one transfer per use year
Note, that you do not have to wait for a your use year to arrive to do a transfer as you can transfer points that are in your existing use year or the next one at any given point in time. Example: assume you have a March use year and you want to make a reservation today for time in April 2017, 11 months form now. You can transfer your March 2017 points now before doing that reservation. In fact, you could also transfer your March 2016 points now and have your parents bank them into the March 2017 use year and use those points for the trip (though you would be doing two transfers at the same time, you do not break the one transfer rule because you can do one transfer "per use year").
Transfer or Do Separate Reservations: When you reserve exactly 11 (home resort) or 7 (other DVC resorts) months out, the online system opens at 8 a.m. eastern, and phone lines at 9. Thus, being able to do it online is an advantage, and, particularly for making reservations for high demand times like the last quarter of the year at WDW, it is an advantage that can mean the difference between getting a reservation and not getting one. If the new contract is in a different use year from your parents' existing use year, the transferred points will not show up after transfer in their original membership account online and cannot be used to do online reservations, but, instead, they must call to use them for a reservation. However, if the points are in the same use year, they will show up in your parents' online account for their original membership number and will be usable for making reservations online. Thus, having the same use year avoids that online reservation problem with transferred points.
An alternative to transfer, is to make two different reservations for any one trip, reserving some nights with one membership account and the remaining nights with the second membership account. You can then have Member Services link the two reservations, which will avoid having to change rooms. Note you need not rush to do that link after making the reservation and may want to wait some time to do it because if you ever want to modify a reservation, such as changing to a different resort at 7 months out, or dropping one night or adding one night later, you cannot do that to a linked reservation but must first have MS unlink them. In the past, there was also an issue that you would need to do a check-out and check-in on the tunover date at the resorts. You would not need to move out of the room but just go to the front desk and do a quick check out and check in, close out and pay the bill on the first reservation and get new keys. We have two membership numbers, and use the linked reservation method, and in the last two years, we have found that check-out and check-in issue no longer exists as the resorts' computer systems can now treat the two reservations as one continuous reservation. Moreover, your keys are now the Magic Bands and thus need not be changed.
Where they have not changed the system yet is for making restaurant or FP+ reservations. For doing so, the two reservations are still deemed separate and thus you have two different 180 day (restaurants) and 60 day (FP+) out triggerring dates for the linked reservations. At the same time, they are also treated separate for the dining plan and thus if desired you can have the dining plan for one of the reservations and not the other.
We have had for many years two membership numbers which are in two different use years. We occasionally use both for one trip and generally use the method of making two reservations that we then link rather than transfer. We have never had an issue or a problem with MS doing so. We have not found that having two different membership numbers created a lot more hassles or a higher risk of disaster with MS or otherwise than having one
Some Legal Issues: putting you on the deed with your parents raises some legal issues to be aware of. Mentioned above is the possible probate issue. In actuality that is one that can be avoided until the death of the last surving owner on the deed if you make sure the new deed names all the owners as joint tenants with right of survivorship. What that means is that if any one owner dies, the others automatically become the owners of the deceased's share and the property does not have to go through probate to be distibuted to others. Thus, probate of the property will not actually occur until the death of the last surviving owner. That will not avoid estate taxes but with today's federal and, in most states, state minimum estate value levels set very high to even have an estate tax issue, it may make no difference for taxes.
Another risk, likely low, is creditors. If you or your parents have financial issues that result in someone filing for bankruptcy, creditors may be able to force a sale of the property to pay off the debt owed by one of the owners. Your parents likely own via a deed that says "husband and wife" now, which in Florida is a "tenancy by the entirety" which is the same as a joint tenancy with right of survivorship but has the additonal benefit of preventing creditors of only one of the owners from pursuing the property to pay off the debt. Your parents would be giving that advantage up by adding you to the deed because they cannot have a tenancy by the entirety with you on the deed.
The value of the property will be considered an asset owned by you. One area that can make a difference involves college for your kids. The total value of the contract which includes you as owner, and not just your perceived share, will be included in your total assets in determining your childrens' ability to qualify for any grants or low rate loans backed by the federal government, and the amount of grants and loans offerred.