Questions about Tasklets and other Bottom Halves

Hey Guys,
I am a few basic and perhaps stupid questions. After doing some
research, why are we using a worker thread for usb
drivers and the like versus softrqs for networking. I am assuming this
is due to the networking stack in need of more
scalability versus USB and timing demands and various things . I am
also curious, not from a theoretical as I am reading
Linux Kernel Development( if Robert is reading this, great job on this
book :) ) and learning a lot, need to brush up some of my
kernel skills as stated. Further more I am wondering when are tasklets
too slow or not scalable enough and we need
to move to softriqs, I am asking from experience and not theoretical
as i can just Google or read  my copy of LKD.
Regards Nick