I would agree if the box time were limited to yellow card fouls. Maybe set up different rules for fouls, identifying which would result in penalty time. That would be interesting.
Players are tossed for red cards anyway (and for 2 yellows in the same game). Putting a player in the box for every foul would really stop the flow of the game.
Good points. The one bad side of a penalty box rule is that we'd see a dramatic escalation of that oh-so-girly-and-gay strategy of having players flopping around on the ground like fish in an attempt to draw a call whenever someone makes contact with them.