One of the nice things about Swordmaster is that you can still do decent damage with a S&B, simply because you can still do alot of spirit damage. From an IB standpoint, it's hard to do much damage with a shield because armor mitigation eats up most of what we can put out.
I'm kinda of the same mindset as far as not going all-out dps. I could switch up some gear and gain another 25-30 +dps, and pick up 8% crit or more, but I really don't feel like it'll pay off. I'll try it at some point as I level, just for something to do, but right now I like to focus on keeping good survivability. As for feeling like you get 'mowed down', well, that's gonna happen at the lower renown rank, especially if you're in a group that's not well coordinated fighting a group that is. Autofocus happens.
Here's how I approach the thinking on the S&B vs 2Hander for tanks in RvR:
A tank essentially serves one primary purpose, and that is to Guard someone else in the group. That might be a DPS or it might be a healer, depending on the group. Occasionally it's even another tank, but not often. After Guard, most tanks have some sort of group utility (like Oathfriend or Auras) and a number of crowd control abilities that differentiate them a bit from each other. If you're doing the Guard part right, and keeping your group utility and CC going, then you're doing your job as a tank.
Typically, the tanks will be the last targets to die in a group, simply because they 'can be ignored' as you've been experiencing, Teth.
So, if you outlast the rest of your group consistantly, while performing your job effectively, does it matter how long you live once your group goes down? Oh, you don't die? Fantastic, but you don't kill either. You're a non-factor. So, in RvR, I think it's better to be able to deal some damage as a tank, because helping kill destro faster is another benefit you can add to your group as a tank. Sometimes it's even enough damage to turn the tide of a battle. As long as you arn't dieing before your Guarded target because of Guard damage, then why not run with a 2hander and break yourself out of the 'can be ignored' category?
At lower RR, having a shield up makes a huge difference in survivability though. Shoot, at high RR it makes a huge difference, too! And it does have its uses, especially in larger-scale battles when throwing up a Hold the Line can really make a difference for the team. It's smart to always carry a shield and 1hander with you if you do decide to go with a 2hander. You never know when Stage 2 will happen and you'll need to tank for the King instance!