Right. Ahem.

First, FFX spoilers:

(SPOILER)I don't think that Fayth have to be summoners. Reasons being: Bahamut's fayth seems rather young, Seymour's mother isn't a summoner as far as we know and guardians become Fayth for the Final Summoning.

Regarding the end of FFX, the party have to kill off the aeons so that Yu Yevon cannot use them to create a new Sin, as he would with any Final Aeon. One Sin is defeated, the Fayth can stop dreaming as the spiral of destruction has ended and Spira has new hope. Thus, their dream ends - taking the Aeons, the dream Zanarkand and Tidus with them.

Now... BIG FAT SMELLY FFX-2 ENDING SPOILERS

(SPOILER)The Fayth reawaken during FFX-2 because Spira faces a new threat from Vegnagun and Shuyin, but are unable to avoid falling under its influence. This is why we re-encounter the Aeons in FFX-2.

Fast-forward to the ending, and Yuna and Co manage to save the day and free the Fayth from Veggie's control. I guess as thanks, they return Tidus to Spira (if that's the ending you choose, of course...). I guess to do so they must continue dreaming.