It's not "by accident". You should never be able to move close to an ennemy without triggering an AOO or being able to disengage as a bonus action jumping like a kangaroo.
What's the point of AOO ? A bonus attack only for the player because it's so cool ? No... AOO are supposed to give melee character the control of a zone on the battlefield. At the moment AOO doesn't give any control the the ennemy because of this disengage + exploit turning arround.
Anyway I'm also +- ok with AOO as auto reaction even if it may not be the best. It's not like if they were a lot of melee ennemies so usually you're engaged with only 1 or 2 at the same time.
As DragonSnooz said it becomes a real problem when it cost a spellslot and use a powerfull spell like hellishrebuke on... bad luck, the goblin that have 2HP left... Warding flare is another exemple to understand how bad the toggle on/off is for reactions.
It will be +- ok for smite even if this toggle is something tedious to manage at each turn.