It's not as though a BB connection is going to saturate the USB bus...
You're missing the point by a million miles ....
(a) You've got drivers and all that
to deal with on USB, ethernet cable is so much more painless and stable. Its also a lot easier to troubleshoot a router based connection because you remove the million unknown factors caused by stuff that's dependent on a PC (is it your broadband connection or a dodgy usb port or dodgy drivers or or or or or or or ad infinitum....).
(b) Hardware routers are more stable... fact because they are not dependent on your PC which might be old / virus infected / in need of software updates etc. etc.
(c) ISPs these days expect you to have a router, their kit
needs to be able to see your line stats etc. in order to make educated decisions about what the best sync rate is for you and a number of other items. If you're using a dodgy old USB modem that's only on here and there, their stats are going to be all skewy and rubbish.
(d) As well as being easier for you to troubleshoot, its easier for your ISP to troubleshoot.
Or turn it off in the router control panel.
(a) If you don't need it, why pay for it
(b) If its there, its at risk of getting turned back on accidentally / deliberately by someone or something.
(c) Its another configuration step, another thing to remember, another thing to take up your time. If its not there in hardware, then that's best.