GIs put your opponent in a mixup situation between a launcher and a break attack LH into full combo, meaning you are in control. Besides after I successfully GId I sometimes like to move around a little instead of going in for the mix, and I can do so quite freely. That's stealing a turn if you ask me.
Regarding aGIs, it's true that most characters have them, but they usually only work on certain attacks (like horizontals and not verticals etc.) and almost never kicks. Also the active window for aGIs is usually much smaller (as I'd like normal GIs to be), making them much fairer. Some characters can get obnoxious with them though, like Tira.