I've also seen a lot of people use ACT to customize their UI info a bit better, the most popular seemingly being Special Spell Timers. TBH, this is something that should be implemented in the game itself (CD/proc icons, bar timers, etc.) but as they've admitted themselves they cannot easily implement while still being console friendly. Prime example of this is the horrible repair UI that still requires people to repair every category individually.
As others have stated many times, it's mostly overlooked as harmless until someone uses it to DPS shame people. At which point the jerk being reported is still not being suspended/banned for using ACT, but for the real reason: being a jerk.