That is somewhat the case. Yes, outside insurance companies ask you to pay for the insurance as soon as you buy it. But most insurance companies will let you change the date of the policy (to cover the new date or new vacation). Some will only let you do it once, but others will let you change multiple times. You really need to ask the specific insurance company what their policy is before you purchase. Also, if you get the pre-existing condition waiver by buying the insurance within the 14 (or 21) days required by the policy, then you usually need to inform the insurance company of the change within that time period (say 14 days after you change your cruise date) in order to keep the waiver.
The thing is, also, DCL doesn't even buy the policy for you until you have paid in full. It's not that they put off billing you, they put off the entire purchase. So you aren't covered for anything until you have paid in full. Of course, the cruise is pretty much refundable until you pay in full, so it's sort of a moot point.
Sayhello
I agree for the most part, unless your focus is making sure you get treatment on the cruise if you get sick, and you have a pre-existing condition not covered by the look-back. At that point, the issue isn't that you do or don't get to take your cruise, it's that you can be treated if something goes wrong.
People need to understand that there is risk no matter what you decide to do:
If you decide to buy from a third-party insurer who has great pre-existing coverage, you'll be stuck with the bill if you don't take the cruise (although, as Sayhello points out, many will allow you to change the cruise once.) Also, you won't be able to apply your cruise fare to a future cruise if your claim is denied. However, if you get that mammogram result you didn't want, you have the choice to take the cruise without fear of medical bills if something goes wrong, and if you are too ill to take the cruise, you can get your money back. You can also insure flights, shore excursions and hotels that you book yourself this way, and you can't through DCL.
If you decide to buy from CSA, it is expensive and the total medical coverage is per group, not per person. It costs about the same as DCL (or did, the last time I checked), but does not have the advantage that, if you don't take your cruise, you could apply your cruise fare to a future cruise. However, this is really your only option if you need medical coverage and have a pre-existing condition
which does not meet the requirements of the look-back period. Sometimes, it makes more sense to take a policy without a pre-existing condition waiver, even if you have a pre-existing condition, depending on how it works with the look-back period, as Sayhello points out. You can also insure your self-booked flights, shore excursions, and hotels this way, where you can't through DCL.
If you decide to buy from DCL, you need to be very careful about changes to your health. If you get the scary news that the mammogram wasn't clear, or you're in for a heart attack, and it is before your paid-in-full date, decide whether or not you really want to take the cruise, and if the answer is "yes", cancel DCL and get CSA--that way, you won't trip the pre-existing conditions clause. You need to remember, anything you get before the paid-in-full date is an uncovered pre-existing condition, unless the "look-back" clause covers it. You also need to be aware that you will need to cover flights, hotels, and self-book excursions separately. However, DCL insurance has the big plus that they will allow you to put your cruise fare towards a future cruise if your claim is denied.
The question is not whether or not you want to take a risk, but which risks you judge to be greater. For my family and me, the risk of high medical bills while on ship is our most serious threat, so we take out third-party policies with great medical coverage. We are less concerned about losing our cruise fare, since we can schedule our vacations pretty easily. If, however, you have a job that can cancel your vacation, you'll probably want a policy that will allow you to cancel for any reason, and Disney will probably give you the best return on that, provided medical issues are not your most serious problem.