mvincent said:The main point of readying an action is take your attack at a later time. If you do not let your player take the readied attack as allowed in the rules, you have effectively cancelled his turn, and that would be unfair (he could've instead just delayed, or taken his normal attack).
If you are concerned that readying an attack can be particularly good at causing a spell-caster to lose his spell, that is as designed. However, a caster can easily avoid AoO's by casting defensively, or just taking a step back. In the scenario you presented, these tactics could entirely void the characters readied action (while still following the rules).
At worst, the spellcaster could take a regular move before casting (still provokes, but no risk of losing his spell during the AoO).
Spellcasting is just an example of AoO causing action, I'm not concerned with it in particular.
I don't want the character to lose his action. I just don't want the character to able to use their action to get effectively double AoO on someone. Sometimes taking an AoO to get in a better tactical position or to get something accomplished is worthwhile. Using the readied action makes this less of an option, beyond it just not making any sense. The idea of reading an action to attack when they provoke an AoO seems to defeat the entire point of AoOs.