why have faces of 4 edges only?
 

This is mainly aimed at Kurt, but if anyone else knows the answer then feel free to give me a reply too...

i see Kurt likes to stress that we should avoid faces that have 5 edges to it or even 3 faces (although unavoidable at times, like he says)
but why? i thought maybe it had something to do with the planarity of faces (btw, wheres the 'keep faces planar' option in maya 8.5?) but then clearly, as i have seen far too often, faces with 4 edges can also become non planar.

so why stress that we only have faces of 4 edges?

When an object is rendered it is actually converted to Triangles by the renderer and if you have four faces then the convertion is obvious (the faces will be split in half) and you know that it will convert following the flow that you have created, but if you have say five sides then it could be split in any number of ways and alter the flow of edge loops giving you nasty looking geomtery and deformation when you animate.
Why not use triangles you ask, well the answer is that you could but it is easier to model sticking ot quads in my opinon.What you want to avoid is ngons (faces with more than 4 sides) the odd triangle especially if hidden will not hurt you too much.

ah i see.

so avoid n-gon faces on n>4 and the render will render in a more expected manner

that actually probably explains why some faces i get on one of my models becomes multi-shaded colours even with a planar face :confused:

It's not necessarily a huge deal in my opinion to have n-gons or tris, but when it comes to smoothing, quads give more accurate results which can avoid pinching and such on the model.