And yet the Jedi shadow has both DPS and tank specialties. I don't think it is a lot to ask. The Sentinel tank track could be focused on using both sabers to deflect attacks.
I personally think the Sentinel is one of the weakest of the classes. Too difficult to do consistent DPS since your DPS is melee focused, and too easily killed. I would love to see Sentinels get a little tougher. Not as tough as a guardian, but if you want to spend spec points in tanking, thereby reducing your DPS, why not?