The harm, obviously, is that the GOP is running against itself for several months instead of running against Obama. Beyond that, there's the money issue, which I've mentioned before.
The political wonk in me would like to see a brokered convention because it hasn't happened in decades and it would be interesting. The rational person inside me knows that a brokered convention is idiotic and harms the GOP's chances to win.
The rational person inside me says there is no such thing as bad publicity and the more free publicity the GOP candidate and process get, the better.