Basically, the reason they have very few tools in game but allow for UI addons is sto stop the single player TES brigade that they've inherited chucking their dummy out of the pram but also allowing for a sensible environment for MMO play to take place.

It is possible for addons to slow your game down depending on: your hardware, the number of addons you use, and how efficient the addon is with memory. Best thing is to just try them out and remember that non-combat addons could always be disabled before you go doing any large scale pvp or similar.