My take on it is that matching use years is a good idea unless you are positive you'll almost always be using one set of points for one purpose and the other set for a different purpose and you won't want to mix points for big trips. If the use years don't match, the system won't let you mix points quickly and easily. You have to switch member numbers (accessible within the online site) to see your different use year contracts, and then if you want to combine balances you have to do a transfer from one to the other. You ostensibly have a limit of one transfer per year, but Disney has generally let people do more than one transfer per year between their own contracts. But transferred points don't show up online, so you just have to know what you have where, or call MS every time you want to check your balances.
If you have matching use years, everything is pretty seamless. It automatically combines your points for booking stuff at 7 months (or 11 months if both the use year and resort match), and you can see all your balances easily in one view.
For this reason, it's a good idea to get the most difficult contract first, to sort of set your use year, and then get the stuff from easy resorts with the same use year. For example, if you wanted to own VWL, AKV, and SSR, you'd want to get VWL first, in any use year that works, and then get AKV and SSR to match. There are so many contracts for those resorts that getting a matching year isn't too hard.
On the other hand, if you want one set of points for one specific purpose and the other set of points for a different purpose, matching use years isn't as important. Personally, I think you're likely to want to merge points at least every once in a while for a large trip or something, so unless you're in a huge rush, you might as well wait for the right year to come along. There are typically around 200-275 contracts that change hands each month (not counting ROFRs), so all use years come up for all resorts within a few months. If you have very specific point size requirements as well, it can get to be a drag waiting.
So if you're buying a resale add-on, you kind of need to be patient or not too picky about price, size, stripped vs. loaded, etc. Remember: you're still saving tons of money. Tons!