The game depends on cooperation between the players and the DM. If someone is playing a Fighter, then the player has an obligation to pay attention to actions that might trigger his Combat Challenge. The DM has an obligation, when the marked enemy takes actions, to allow that player the opportunity to use his power correctly (and not simply announce a successful hit and damage).
If the DM's going to choose to run his game in such a way that an immediate interrupt will be useless, he needs to let his players know that before they generate characters, and also make sure not to use adversaries with immediate interrupt powers.
If a player is going to pick some immediate interrupt powers, he takes on an obligation to know how and when to use them.
We have a Stormwarden ranger, and I don't go back in time and apply his End of Turn damage if he forgets to do it. He chose the paragon path, and it is his responsibility to announce which (if any) opponent takes the damage. We also have a wizard with a Staff of Defense and a Halfling with Second Chance. I know (as the DM) that the characters have these Interrupts, so I deliberately give them a bit of a pause to use their powers at the appropriate interval.
It doesn't take that long to allow decision-making (or if it does, that's probably a problem with the player making decisions in general), and there's no contest to finish the game in the least amount of time.