As computers get faster and faster, this taxing that usb causes becomes less and less of an issue. It kind of like asking why anyone ever uses onboard audio as it taxes the cpu as well. Back in the day it was very bad and even caused lots of skipping on loaded systems.
Now days USB does not have much issues. I do remember having a Celeron 466 and on usb the mouse would skip under certain situations. I used PS/2 for years for this reason. But my last few systems have used USB for mouse/keyboard since those problems are gone with modern hardware. A side effect of this, I get to use my mouse with 1000 report/sec(i do notice this difference, but only up to about 500).
Firewire while invented mostly for DV work, has been almost fully abandoned. Even Apple does not include it anymore on many systems. And they pretty much invented it.