Yeah, gster hit the nail on the head every time there. The other thing about triangles is they're much, much easier and hence quicker to render. On a side note, it's not possible to have a non-planar triangle. Non planar polygons are a _real_ pain to process and light correctly.
There's just a hell of a lot less maths behind doing the calcs for triangles. As well as mental ray going a bit mental
at some dogy subd topology converted from triangles, subd meshes are much 'lighter' and easier to manipulate and edit than ones that've been made from a mesh containing lots of triangles. Subds are also a much more powerfull way of editing, having the ability to affect areas with a weighting function, far better than say the sculpt tool does.
Trust me, write a scan-line renderer that will only handle triangles, then write one that will handle N-gons, with an upper limit of say 20 edges. As soon as you've done that, you'll realise just how lucky we are to have maya churn through almost anything you can chuck at it. Ngons are not fun to program for.
It can be well worth blocking out in quads, converting to subds to tweak and finalize before being converted back to polys and onto tris for a game engine. It's a very quick way of working. Just ask Kurt.