I don't have a strong opinion on the topic (I never really bothered to look under the hood but from a purely user perspective USB has been quite serviceable for me so far) but I think you're begging the question:

>You don't have direct interrupt lines like in PCI.

Surely the people who've designed USB could have opted to add an interrupt signal if they had deemed it necessary. Sure it would've added one more signal but given that there were only 4 pins on the original USB it doesn't seem overwhelming.

That being said I agree with you that dismissing polling wholesale without digging deeper is quite silly, it can be a problem when it increases latency or wastes CPU cycles but as far as I know latency isn't much of an issue in most uses of USB (even for things like keyboards, mouses and controller it's usually dwarfed by the latency of the video pipeline) and I doubt anybody handles the USB phy in software so CPU usage shouldn't be a worry.