If you listen closely to the guitar parts in Primus songs, you'll realize that his guitar style stands out from the crowd in the same way Les Claypools plinkity-plink bass playing does.
He's not like a lot of "backseat" rock and metal guitarists I can name( that guy from Nightwish, Malcom Young, etc) who just chug along. Unlike those guys, you'd actually notice if he wasn't there.
The other reason he's awesome is this: he has chops, and knows when to use them. He's one of the few guitar players I can think of who knows the meaning of the phrase "tasty playing", and doesn't just masturbate all day( Joe Satriani I am looking at you, ya bald-headed wanker!)
And Primus kicks ass.