"Primary" is misunderstood. It means the active class that you're playing, with all of the available skills for that class. "Secondary" means the second class, which only has that class' General skills available to it. There can also be a Tertiary (3rd) class, but it is always completely inactive.
M/P (or P/M) is always a good choice. I've never played a P/R or R/P, but there are a number of people who swear by it. On the other hand, there are a lot of people who swear *at* the P/S--especially in Siege.
From other threads, I'd recommend avoiding P/W, the Battlemonk. They're not easy to play from what I've read, and other class combos either do far more damage and may be more survivable.