I don’t use addons to change the default layout of how blizzard presents where the menubar etc is.
I use addons for things like raids/dungeons, and probably the two I would have a hard time playing without would be AH addons and pet battle addons. I don’t want to remember which pets to use to fight certain pet bosses, so the addon that lets me save all that is absolutely gold. I would give up doing pet battles without it.