AC-DC adaptor to run case fans

[Bean]

I'm using a 120vAC to 9vDC adaptor to run a couple case fans. I have em on top of my monitor sucking out the hot air at the moment. So far it seems to work. I'm wondering if anyone's tried this on their PC? If you have a plug output on your PS it will turn the fans on when you turn the PC on.

PRO's
Adaptor gets warm but it will be outside the case so no problem.
Less load on the PS=less heat output=ability to use slower fans to cool PS.

CON's
duh, help me here. I cant think of any really.