Except it's commercial, incredibly overpowered for anything 2D, and known for having some pretty bad bugs.
Ah, but it usually makes games a lot more popular, just look at Skyrim/Fallout 3 and Half life 2.
You wouldn't be able to enjoy those games the same way without it... Glitched physics engines are more fun than the average run of the mill engine. And to say you cant use a 3D physics engine on a 2D game is kinda dumb as well... just don't use the Z axis... better yet... use it to your advantage(Think Little big planet and how they switch layers). If I had the money and time I would totally use Havok over Box2D just for my 2D platformer, because I can. Also if the game is good and you have made lots of money off it you can flop to 3D, making Havok better taken advantage of. Doing different things on your games makes it unique and more fun.
I hope I made some sense.