I like it. ^
They make the gate charge painful enough that people won't want to do it. That lets them process the payments when people are checking in, instead of in the time leading up to departure. That frees up their agents to work on other tasks.
I think other airlines could really benefit by clamping down a little more on their carry-on policies (maybe not to the level of charging for all carry ons). Stop all of the morons from dragging their huge bags down the aisle, or letting them gate check for free. As long as they are clear about it, and there are baggage sizers at the check in counter, people shouldn't complain. They will, but they shouldn't.