I think I summed it up a while back. Look at the reasons people buy these things. They buy them as a portable PC for LAN parties, but they don't even have room for some video cards, and lack the power needed anyway. They also lack room for drive expansion. A Micro ATX tower would be a far better choice for gamers.
Other people use these things for HTPC. Well, to begin with, there are several ATX and Micro ATX case that are actually made to LOOK good in a home theater rack, being about the same size as a home audio receiver, and having the styles to match. Additionally, since the width of most home theater racks is fixed at around 18", the additional HEIGHT of the narrower cube (which looks stupid sitting with your home theater components anyway) causes it to use MORE space on the rack while offering LESS internal space for components.
So the only two "good" uses for cubes are actually better served by Micro ATX.
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>