I think people are being a little unfair to Ajax.
Better graphics and hardware give artists more freedom to give life to their artistic vision. I could just imagine how frustrating it would be to be an artist wanting for a game to look just perfect but to be hindered because the machine was not technically able to fulfill my artistic vision. Maybe all the little touches that suck up processing power don't matter to some people. Maybe flowing hair and rippling realistic water aren't important to you. But consider it from the perspective artist would love to see her creations come to life in vivid beauty.
No one "needs" anything, but it's better to have lots of capabilities in the machine and not need them all than to be limited, restricted by what the machine can do. If Square feels that the PS3's hardware is most able to fully convey their artistic vision to the world then that's what they'll use (within reason, budget...etc).