I'd like to know what the real typical power distribution is in a system. What draws how much from each supply voltage. I might have found something on it somewhere, maybe Anandtech article on PSU's. Maybe it really doesn't matter if 5v and 3.3v are shared total. I think 12v powers all the really big loads anyways. I think it goes like this:
12v : all drive motors, mobo Vcore supply, cooling fans
3.3v : ram, northbridge, GPU, misc other mobo chips
5v : other mobo logic, all drives logic, USB, KB, mouse
Of that, 12v is the vast bulk of the power. The 5 & 3.3 are not that much, and things like chipsets, mobo peripherals would be one or the other, not both. Frankly, I fear for cooling the system if 5 & 3.3 pile up to 175W. The 12v loads are enough heat. Many drives plus externals like USB/KB/mouse could pile up the 5v a bit though, so I'd hope it's got more overhead.
On the other hand the manufacturers could put in two whole low V regs., one each for 5v and 3.3v. Maybe a good idea in extremely loaded systems, but is probably overkill/$$$ in most simple boxes.