Most class combos have one side that works better as primary. K/w and WD/W for tanking. You will see a lot of R/M, very few m/r except when leveling secondary. R/M does significantly more damage. P/W is an interesting and fun questing class, not so much w/p. I am told that S/R is better than R/S, haven't played it. M/WL has cool boss debuffs that make it handy for instance running, whereas some groups will also include a WL/M for the group buffs. It goes on and on.