I can't speak to the balance existing at this time, but I'd imagine the fact that haste has been given to Arch-Mages in nearly all games played into it.
3 base attacks is the maximum any player can get to by any means. After combat rounds have been established other effects can increase this - harah lit rune, criticals, extremely high skill, etc. Recently I've done the tests on average attacks per round, and fighters using whirlwind are one of the, if not the single highest average attacks per round.
I've done no checks for duration. Nor have I spent any time determining the effectiveness of skill level for whirlwind attack in particular. If pressed, I would venture a guess that raising skill raising duration is enough of a boost that raising effectiveness further beyond the ceiling for attacks per round was not thought of as important.
