The Lix buzzsaw hazard is probably a good example of where a non-rectangular trigger area is desirable (namely circular, obviously). I'm always a little bothered that with the current trigger area, the lix can be closer to the buzzsaw than the visual graphics would imply, but of course with a rectangular trigger area and a circular visual shape, you just can't win--the trigger will always be too large or too small.
Looking beyond that, having other-shapes support could allow, for example, versions of the Lix laserbeam hazard that can be at any angle, not just horizontal or vertical.
So as you said, it's not that we want to promote crazy shapes, but clearly there are at least a few good shapes, and being able to support more arbitrary shapes on the outset would prevent needing to add more supported shapes in future updates.
I'm less convinced of the need to allow multiple-disjoint-regions trigger, mostly because in most cases it seems like you can achieve the same effect via multiple objects, each with their own continuous-shape trigger, that can be put together to achieve the effect. But for that same reason (ie. seems like it can be emulated anyway even when disallowed), I'm not opposed to it.