There's actually an even better way to do it - disconnect the USB plugs on the motherboard and connect pins 1 and 4 (the outside pins) directly to the PSU's 5V (red) and Gnd (black) lines.
No, that's just as bad. USB is spec'd to provide 500 mA of current. USB does include power management to manage and restrict power draw to devices, but because people are incredibly lazy, not many devices really use power management and just draw power from USB nonetheless. This seriously messes up every power management aspect; there shouldn't be just power on the US bus for you to use; a decent USB device uses little power for startup and then requests more power, which it will then be allowed to draw depending on the current state of connected devices. Just using Y-Plugs totally counteracts these measures, as noone knows how much power is taken from where, so decent power management becomes absolutely impossible.
This leads to lots of stupid situations where devices connected to the US bus stay on even when the computer goes into standby; USB can, by specification, signal the devices to go into standby, but since many of them don't use power management, they won't.
The only problem of the USB spec is 500 mA isn't exactly much. 1 A would solve many problems, but just doing stupid non-spec'd things will only make the situation worse.
Therefore, having a bus-powered hub with a socket for a small power supply (doesn't need to be included, just write the plug's polarity and voltage on it and you're done) is the best option.
Screw the specs though, the main reason USB ports are limited to that amount of power is because if you've got a cheap-o PSU that has a lot of noise on it's 5V line (like that bestec crap in almost all pre-built systems), too much current on the USB PCB traces will introduce that same noise into other lines on the motherboard. Interference within the cable itself won't make much of a difference unless the noise is really bad or the cable is way too long. So bypassing the motherboard or splitting the power draw among several ports gets rid of this problem, and actual cable interference and heat becomes a problem a lot later (in comparison).
The main reason the power is limited is because no one would have thought people would start connecting their hard disks to the US bus. They should have remedied that with specification 2.0, but since they wanted to stay backward and forward compatible, this wasn't really an option (they could have introduced a mode for drawing 1 A, though).
-huha
