There might be some use-cases out there for not wanting to do this, When I merge one branch into another branch, if I make a change in between finishing the merging and checking in my changes, that change becomes very difficult to trace as to when it happened. If that change breaks something it becomes nasty to track it down and then revert it.

There are probably use-cases where people wouldn't want that behavior but I'd like to be prompted to go to pending changes and check-in my merge once it's finished.