My understanding has always been that degenerate facets are those with less than three verticies (a point or a line, instead of a triangle or a quad). Collinear facets are those where the first three points are in a line. This seems to be how UV Mapper Pro looks at things. I can certainly see how one might lump them all into the degenerate category :-) In any case, the reason they are bad is that most renderers use the first three points to determine the normal vector for the facet. If the first three points don't make a good triangle, it has no way of knowing which way the surface faces, so you get a bad normal and it looks black.