They are a tank and as such, their "main" stat will be VIT.
There is the STR route that many endgame tanks like to transition to once their gear is up to snuff.
The value of PIE to a DRK is still untested, it may be excellent and we could see full PIE builds or it could be worthless where a proper rotation makes the extra mp gained moot.
I personally will be going full VIT until I can see more data on the STR vs. PIE benefits.