Quads might be a useful primative.
One might argue that quads can always be turned into triangles, however, this throws away information that might be useful. For example, when generating vertex normals on a patch you get better results from a quad tesselation than a triangulated tesselation. Also, Kobbelt subdivision on a quad patch would be better than refinement on a tri mesh that gets biased by the choice of diagnol.
Collada already enables quads to be represented by using polygons. However, somethimes there are a lot of quads. Having an xml node for each quad makes the file heavy. If you pop it into Internet Explorer to eyeball the file, it will load really really slow.
Unlike tristrip, quad could be a justifiable primitive in collada.
Just my 2c