Because it creates some nasty corner cases, that are ripe for abuse. I throw an attack, that provokes an OA. Opponent takes the OA. I use an interrupt, that invalidates the OA. I attack with the interrupt, then attack with the attack that triggered the OA. Opponent has already taken his OA on me, for that turn, so I get two in as freebies. It's cheesy.
I apologize if I'm thick, but I just don't get it. Perhaps if there are other examples than the Disruptive Strike example? Disruptive Strike appears much better used during another creature's turn than it would be in the ranger's turn.
We agree that the ranger has an immediate action here, that he'll be using anyway. The question is only:
when does he use it? During his turn, on during the monster's turn? Based on this, this is what I understand:
Case 1 - RAW
Ranger's turn
Ranger attacks
Monster's turn
Monster attacks
Monster triggers ranger's immediate action
Ranger possibly invalidates good attack by monster with his immediate action
Case 2 - house rule where immediate actions may be used during ranger's turn - possible abuse??
Ranger's turn
Ranger attacks
Ranger voluntarily provokes OA by monster with attack or movement
OA by monster, that triggers immediate action by ranger
Ranger possibly invalidates OA by monster with his immediate action
Monster's turn
Monster attacks
So... Where is the abuse? In case 2, the monster gets one extra attack, his OA, that is voluntarily triggered by the ranger; while in case 2 the ranger can invalidate a melee basic attack only and not a good encounter power or some such by the monster. If anything, I think the ranger is taking the wrong decision, by far, in using his Disruptive Strike during his own turn.
And then you have the issue, that it simply adds another layer of complexity to single turn actions series.
I don't see an added layer of complexity based on the fact that the ranger can use his immediate action anyway. Either he uses it during his turn, making his turn longer; or he uses it during the monster's turn, making the monster's turn longer. The only added layer of complexity is the additional OA by the monster, which the ranger decides to voluntarily trigger, giving the monster one extra attack during that round.