The scaling on the "increased by attack power" is designed to work with vengence. Vengence adds MASSIVE amounts of attack power. If it scaled as quickly and easily as a trinket use, raid tanks would receive massive benefits to survivability. Unless they changed something, vengence does not work against players. That leaves very little options for a significant increase in SB power in pvp. I'm sure that's by design so that you will not be invincible against casters. Spell reflect, SB, stuns, etc. prot has a lot of anti-caster tools. An immunity might be asking for a little much.
For pve, generally you will be (or want to be) at about 3 parry to 1 dodge. The way the diminishing returns works that maximizes your actual gains from the rating points you have invested.
The goal of prot in pvp is not to actually kill anything. The goal is to be as annoying as possible. You want to frustrate the other team into attacking you and wasting a lot of effort to control/kill you so that the rest of your team almost gets free kills. Charge stunning, other stuns, gap closers etc are all just ways to try to force them to train on you. Once they do, you should be much harder to kill than the average pvper which is to buy more time for your team to freely act. By the way, if your team does get them low, excute still hits like a truck.
Prot is a different mindset in pvp. Some really enjoy it and others, not so much. The glory won't be in damage done or kills, but more so in trying to control the other team and frustrate them. The better you are at that, the more successful your team will be. For that reason, prot is more effective in 3s and 5s. 2s require more contribution from both members and the loss of damage makes it harder for prot to do it's job.
Edit: I appear to have responded to a post that was deleted while I typed my response. Hope it helps someone anyways.