The extra Aeons are WELL worth it, and not nearly as difficult (or as time-consuming) as fully activating all the legendary weapons. You absolutely cannot miss out on such Aeons as Yojimbo, Anima, and The Magus Sisters, IMO. You might need a little FAQ help to get them, but they RULE. Anima' s Pain is just one of the coolest damn things ever.
If you didn' t know, fully activating the legendary weapons requires that you have both the Crest and Sigil for each (i.e., for Tidus' Caladbolg, you need both the Sun Crest and Sun Sigil). The Crests are pretty easily found, but the Sigils are the tough ones. For example, to get the Sigil for Lulu' s Onion Knight, you need to dodge 200 bolts of lightning without getting hit once or stopping to save. And this isn' t even the toughest challenge...
Once the weapons are fully activated, though, wonderful abilities get added, and they include an ability that lets every spell cast cost only 1 MP (yeah, you heard me), and the Break Damage Limit capabilities as well (these allow you to deal in excess of 9,999 HP damage).
In my game, Tidus and Auron generally do between 15,000 and 40,000 HP damage per regular strike, and Lulu and Yuna do about equal with their spells and summons (Aeons). Of course, the Aeons need the Break Damage Limit on THEM for that to work, so it' s a bit tougher. There are also the Break HP Limit abilities, so your characters can have more than 9,999 HP. I think most of my characters had between 12-15,000 HP. It just gets crazy after a while. LOL