Not sure what your settings are (or what version of Maya you're using), but Maya does fill up holes in the UV while packing, just as you describe. Bad example below to prove it, and my settings:
The UVing itself is great. I have no issues with Mayas UV tools, its just the packing I cant ever to get the results I want. It doesnt fill any empty spaces UV shells might have (like a tire with a hole in the middle for example) so there ends up being tons of wasted UV space. I just always seem to get a much better pack…