Some addons add performance over the default UI, especially those involving the inventory. Others are going to remove a lot of clutter or other ui nonsense (such as using dominos for the actionbar), adding some near essentials like healing comm others will do, etc. A lot of load on demand is going to fit into this performance as well

I have my slaves at times do some trading between each other. I need a small addon to toss items in the trade window based on what is setup for that character. Same for tossing stuff into the guild bank. Sure thats not needed 99% of the game, good deal its not loaded 99% of the game either.

I use pitbull4 since the plugin status is pretty good, so like people use for jamba the follow status I have an icon on each of my unit frames for follow status and who they are following, a crude direction facing indicator (sadly non instance), and party ability bars (omnicc for on your main) like cooldowns. I like to use Parrot quite a bit for its display regions (one for each character) to show special alerts (if they are too idle, showing their throttled errorframe details which usually means they didnt recover from a kick or fear correctly and need my attention). I need a client side piece for that stuff to get over to the main efficiently.

I also like to auto-buff my guys, that will require an addon. Not doing so seems pretty harsh.

I also like the ability to swap any of my characters to be the leader of a group, so some things that you might want to see as the slave might be nice to have there if your doing the pip swap.

But if you look the small garbage collection these addons do is not much at all. If your running a carbonite or an autobar, well then you can run just about anything at that point