I may be totally wrong here, but you may be confusing patches with channels. For light maps, you typically put it in a second UV channel that still exists in the 0,1 space. With UDIM, the point is to have one object take up several UV patches so that all the polys receive high resolution textures, rather than cramming them all on one patch. So using the wrap index function would get a whole new texture set, as each patch uses the same UV’s with different textures, rather than texturing the whole object at high resolutions.
Please correct me if I’m wrong, I want to know. I’m not a professional texture artist, so my knowledge is just from the past few years of research.