The value of longer contract duration is either 1) more nights for you to enjoy at Disney or 2) more nights for the person you sell your contract to in 10 years to enjoy Disney (therefore increasing resale value).
On the other hand, longer contract duration does come with more risk. We like to think that Disney will continue to be the powerhouse tourist destination that it is today indefinitely, but if this ceases to be the case, you could be stuck paying maintenance fees for much longer on a less than desirable location (similar to traditional timeshares). FWIW, I don't actually believe this will happen.