I don't actually know that about DL for sure, I've never tried to book anything other than dining in advance... I assume there are some other offerings from time to time?
First of all, I assume I'll always have at least 2 UY because I have two separate direct contracts and they'd be the very last points I sell, and I'm going to have 3 UY until BCV expires or my family stops liking WDW, whichever happens first-- so I do not judge anybody else who has 3/4/5/6/+ UYs... however, knowing what I know now, I'd be more careful about keeping all my WDW contracts together so that I had the option to stretch out 10+ day trips without needing to link or transfer. Transfering seems much riskier to me because it then limits your options to move between UYs. Due to an inefficient marketplace, the costs of swapping out one UY for another are very high so I don't plan to move to consolidate.
On the one hand, you could argue it makes more sense to keep the California and beach resorts separate... but if you think there's any chance you'll ever want to use some home resort points as SAP for a big/long reservation, you're still better off keeping them in one UY. For example, I may end up using BCV, VGF, and AUL points all for one WDW trip this fall. I'm staying at different resorts so they'd be linked instead of consecutive in any event BUT I did actually have the option to extend out BCV and would have done so if I had all my points in the same membership to give myself the most options to book in advance at the busiest time of our trip. At least, I think that's how it works, since I don't have any UY with multiple resort contracts.