I think it's just about usability - as far as I know, the game environment treats everything as tris.
SU doesn't export models in a nice, quadrilateralised form, since it was never made with quad-based modelling in mind. A face with some rounded corners, some holes cut, and maybe some arching edges may seem simple in SU, but actually translates to a lot of tris that export in whatever arbitrary way SU decides, making it difficult to edit in Maya, Max, Blender, etc. Quadrilateralising the mesh helps, as it arranges the tris nicely, but even so, they are still tris.
The main reasons I like quads:
-Easily editable. Dragging vertices around on a quad mesh creates predictable results. ngons act strangely in this situation.
-Edge loops. Edge loops are a huge part of most modeling applications, and allow for easier selection, division, and readability. tris and ngons do not produce good edge loops.
-Deformation. Quads deform predictably and smoothly, making them much more presentable in an animated mesh.