I have had ongoing problems with the primary clutch on my 2004 Summit 800. I have been destroying belts and already spent $250 to replace all the rollers, washers and sliders (buttons). I notice that the the sliding half of the clutch actually runs into the fixed half and is causing damage at the contact point. What is supposed to prevent this interferance? Is there a stop or is the spring supposed to hold them apart?

This is normal on the primary clutch. For 2006 BRP was putting an oring type of washer between the 2 halves to stop this. I don't have the part number handy but any dealer should be able to get one for you.
As for your belt issues the number one killer is improper alignment and the second is heat.
You can use the alignment tool from any dealer or use a piece of 3/8 square stock or the best yet is to purchase an alignment tool from Sled Pro
http://www.sledpro.com/page38.html
Even if the dealer claims the alignment is fine buy the Sled Pro tool and use it.
Once they are properly aligned make sure you have removed the black plastic clutch gaurd. It hold in way too much heat. Remove the whole tool box assembly from the footwell. Use a hot glue gun and put some screen in any open vents on the side panel to keep snow out. SLP I believe it is makes some vents that go in the rear of the shock tower.
http://www.startinglineproducts.com/catalo...1&productID=612 (http://www.startinglineproducts.com/catalog.cfm?pageID=detail&catalogID=4&catID=31&productID=612)
This will get lots of air flow and temps will go down.
Lastly a good after market roller secondary really helps. I run a Hyper Lite from PEM , and although they are out of business now, my primary and secondary are so cool that I can not even warm my hands off of them. Even after back to back mile long pulls down the lake.

I think the belt is supposed to prevent that. Mine never had the smash marks on the center of the primary sheaves, until one time I was revving it in the garage without the belt on (didn't really think about it, I was trying to burp the cooling system) and I could hear it smashing together. I looked and you could see the marks from where it was hitting. Now I try to make sure if I'm going to run it without the belt I keep the revs below engagement.

Thanks for the input. It has been a head scratcher for me lately!

Sheaves touching and marking is common on low geared machines like your summit. Usually happens when your full throttle climbing and you get small air or hit some soft, clutches upshift fully. Also happens on the stand with the belt off.