We have one very problematic slot as pugging tanks, and that is the helmet. While pugging Cata tier one raids is perfectly doable, a 359 helmet isn't all that much of an upgrade compared with the 353 one that eventually will drop if you farm the trolls for Valor Points.
So we need to find us a replacement. That replacement would be the 384 PvP helmet. Now, a PvP item doesn't sit all too well with most of us, but this one comes with a bucket load of mastery. The Resilence is, obviously, a dead stat, but at ilevel 384 it comes with a big chunk of armour and stamina to make up for close to 300 points of wasted stat.
Now, I'm not even remotely arguing in favour of this helmet compared to a 378 PvE one, but that's the catch. You need to pug down anything dropping a 378, and that might prove difficult for a casual tank. So we're stuck with a 359 or a 353, and compared with those the PvP piece doesn't look too shabby.
Now we have another problem. 2200 Conquest Points. Doing arena as prot is more or less out of the question. I've spent a lot of time dying to cram out a few wins, but you'll make yourself an unhappy partner.
Pugging rated battlegrounds, however, isn't entirely futile. A tank is quite often handy there, so you can pull your weight properly.
Still, you may be a casual tank for the simple reason you can't spend a lot of time gaming. Then you're unlikely to want to spend the time you have available smacking other players over the head.
So your last resort would be to convert Valor Points to Conquest Points. At least it's done at a ratio of 1:1.
I'd suggest that you wait with it though. There are quite a few items which should come higher on your list, but eventually you're going to run out of truly appealing items to buy for VP. That's when this helmet, and of course, the PvP cloak starts to look appetizing. The helmet because it's the best we casuals are likely to lay our hands on, and the cloak because it's the second best there is in the game.