Sin and Zanarkand SPOILERS

[Jiro]

Yeah, if the fayth were the only thing keeping Jecht alive, and their purpose was to defeat Sin, then surely stopping their dreaming at any point would've been fine.

A fayth CAN'T choose to stop being summoned. They granted the summoner their power, and ever since then that summoner can call forth their aeon.

Bahamut's fayth asks Tidus to "let them rest, because they were tired of dreaming", remember? This very much implies they cannot simply do it themselves.

So did Dona and Isaaru have to be tracked down before Yevon would smurf off? What about Brother Zuke who was a summoner previously?