I think most people start using slaves for the extra RAM -- or at least that's the way it was for me. I see you use Sonar, as do I. One advantage to having all of my instruments on slaves and keeping only Sonar running on the DAW (master), is that when Sonar does it's automatic save every few minutes, it does it lickety-split (Southern U.S. for fast). If I have a bunch of vst and samples on the main computer, it can take several long seconds to save -- which can get annoying.
Also, another reason to have slaves it that you (or at least me) don't have to upgrade them quite as often. If you've got a reasonably powerful processor and 8 or more gigs of RAM, that slave is going to be good to go for a long time to come.
Thirdly, I suppose having slave machines just keeps things tidy and neat -- though I have paired down to two slaves only. I find that three or four slaves gets to be a headache keeping up with operating systems, etc. etc. Ultimately, I'd like to go with one 24 gig i7 slave, but that will be sometime in the future for me.
Good luck,
Mahlon