If you have a lot of good cross class skills you'd be better off playing MRD for solo play. All of the Warrior skills are tied to Defiance and keeping up Defiance while playing solo will slow down your kill times.

That said you still get stat bonuses from simply being a Warrior so even if you don't have Defiance toggled on it's still worth staying as a Warrior, provided you're happy with having a limited pool of cross class skills.

As for tanking you don't really ever want to do it as a MRD once you hit 30. The benefits from Defiance and the rest of the Warrior skill set are pretty big. Brayflox's Longstop is where these things start to matter much more.