If you're on a Macbook, the largest number of USB interrupts is probably from the Bluetooth controller. Either hciconfig hc0 down, or load hci_usb with the isoc=0 option. The other Macbook issue is that the trackpad will carry on sending updates even though there's no input - I wrote a patch to fix this, which ought to be in git now. These have nothing to do with the UHCI spec.
Why are C states so important? They allow the CPU to turn parts of itself off. However well designed your CPU, turning parts of the processor off is going to save power. Of course, this can only be done when the system is idle (code doesn't run so well on disabled execution units). It's also inevitable that there's going to be some latency in reenabling these portions of hardware, so you only want to enter them if you know that the processor is going to be idle for a significant (like 20ms or more) amount of time.
In C4 state, a modern Intel CPU will draw somewhere in the region of a Watt. There's absolutely no reason why a modern desktop shouldn't be spending most of its time idle, so we should aim to be hitting C4 as much as possible. It's low hanging fruit, and Powertop lets us identify most of the things preventing us from getting there. Processors are always going to be able to save power if they're idle. It's an entirely worthwhile project.
For what it's worth, the ULV Core 2s will max out at about 10 Watts, which isn't bad - sadly the Macbook Pro contains a T7600, which goes up to 34W. The last-generation Powerbook G4s with 7447As drew in the region of 20 Watts - Apple seems to think its customers prefer performance to low power consumption, and I don't think you can blame Intel for that (especially since you can get an LV 1.6GHz Core 2 Duo with a TDP of 17W, giving you rather more performance than the G4 Powerbook while consuming less power)