Well, it wasn't really the last,Yu Yevon was. Also, without it you might be f*cked because your aeons level up as you do, but faster. Like if your hit and HP are maxed out your aeons can still instant-kill you.
Although when I've played I don't think any of the aeons did anything major that would kill everyone right away... but I'm not sure because my stats are high so maybe in a normal run though the aeon battles might be a huge mess without auto life.
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