I don't think it is so much of "are they really worth it?" Moreso, I think it is what is more cost effective? If you plan on doing a theme park for the majority of those days--then a length of stay pass or individual park hoppers are going to cost gobs of money. It would be cheaper to buy the annual park pass, and then perhaps pay individually for things such as Disney Quest, Pleasure Island, Typhoon Lagoon or Blizzard Beach.
If you are looking economics--figure, how many days in the parks would make you happy, do you really need the water parks, et cetera. Those plusses would be included in your length of stay, but not a annual pass.
I agree with RC, even if you dont use the pass again before it expires, it is so much better.
Most Floridians would purchase a discounted AP or Season Pass (if they are smart) before ever even buying a park hopper that costs more than an AP. Park hoppers are too expensive for us--so if we go for multi-days, it is an automatic AP or SP.