That's a great point. I probably wouldn't do it with a HTML5 shooter with hundreds of projectiles though... what I mean is, those engines still have to check "what shape is it, oh, it's a circle, so now check what I am colliding with..", while you e.g. know the projectiles are circles and the walls are orthogonal planes. This may not always apply, of course, and I agree that as soon as you enter more complex territory you can save yourself a lot of pain by using a "proper physics engine".
> what shape is it, oh, it's a circle, so now check what I am colliding with
While this is true, consider that the huge library of built-up optimizations in a well used collision library like Havok very likely makes up for these checks.
Alternatively, they generally offer a lower level API such that you could just always call: `Havok::CheckCircleAAPlane` and then not have to worry about Havok doing it's dispatch. But then again if you don't use Havok's spatial culling you're likely missing out on a whole lot of optimizations, and advantages to using Havok.
