I'll try an explanation with my situation.
I have no USB equipment but I do have USB hubs active on my computer. They are on the same IRQ as my sound card. Now usually you don't want anything sharing IRQs especially with the video or sound cards because you want maximum performance from them, ie: using their IRQ when ever they want to. If I did have a USB thing doing stuff, like a joystick, the joystick would conflict and delay response from the sound card or vice versa.
My assumption is that if nothing is on my USB hub, it will never request an interrupt that would hinder the performance of my sound card. If I did have something on it they would have to squabble over who uses it when.
Clear as mud now, right?