Instead of something that just happens, every combat, every time, consider a reactive ability that either side is capable of using.
Flipping the 


Out: once per combat, any group of allies in that combat may choose, as a group, to Flip the




Out. To do so, half or more of their members must be bloodied, or one of their members must have been reduced to or below 0 hit points. When they flip out, they have a +2 bonus to attack rolls and a -2 penalty to all defenses.
1 - either side may choose to do this, but it's a personal decision for that side, and is equivalent to giving everyone a +2. It can be up to a +4 for each side.
2 - It is activated in response to a meaningful event. The party generally doesn't get increasingly aggressive and reckless when they're clearing out a bunch of weak enemies, but it will be an option available to them in the big-time fights where it means something.Sure, they'll be able to "cheese" it for their big-deal dailies, however:
-It'll generally only happen in meaningful fights where they're seriously threatened. The penalty will actually be a concern to them.
-The cheesing to ensure dailies hit will actually make sense in character. Playing more cautiously, and then when they see the chips are down, fire off the big attacks!
-Hitting with dailies is fun! Missing is depressing.
If for some reason one of the players thought that wasn't in-character for them and didn't want to participate, you're already houseruling, so there should be no problem allowing them to say they have a cooler head, and they NEVER flip out, always fighting in a balanced, calm manner even when the




has hit the fan.