Um, you can use filters on "network computing", too, and in that case, the data never even goes over the line to your terminal, whereas in the "machine on my desk" case, you still receive it, only to junk it.
It still gets sent from the Comcast server to the server of your network, though. YOU (or your company) still has to do the job.
Good point. I use MailWasher, myself. It does a great job of pre-managing spam for me.