You should not need to download "Ace3" as a standalone addon ever. It is designed to be embedded, so that even if 59 addons embed it, you would not suffer much performance/long loading issues.
However, if you are really asinine about it, you can definitely disembed all 59 copies of Ace3 and just load one copy of it. Just keep in mind that Ace3 is really a collection of some 18 mini libraries, and most addons will only use about 8 or 9 of it (the more common ones), and installing Ace3 as a standalone addon will load all of them.
The only advantages of installing Ace3 standalone:
1. If you really care about loading times and want to disembed everything.
2. If you want to know how much memory each addon is really using, outside of the shared libraries that embedding will distort.
3. You can use /ace3 to show a config screen with all your ace3 addons that have registered aceopt tables (much like RockConfig).
There are only a handful of Ace3 mods in "release status" at the moment:
GatherMate (and accessories), GatherHud, Routes, Mapster, BetterInbox, StatBlocks, CombatDnD and maybe one or two more I missed.
Upcoming addons in Ace3 near ready would probably include:
GuildCrafts, Bartender4, oRA3, kgPanels, Grid, VisualHeal, RaidCast.
Proximo version 2.0 is getting close to release on Ace3 also. All initial coding is finished, but testing is about to start. Still have a few Ace3 minor wrinkles to iron out also (can't get cmdHidden to work, etc...)
I agree with using what addons suit you. I just dont want to miss a possible new addon that I may like more. It would be nice to have a way to see new addons regardless of them being Ace3 or any other framework.
Sort addons by revision number, and look for names you don't recognize. Or look at the changelog on fisheye and search for "initial commit" - that's what most authors put in their notes when adding a new mod to the svn.