I wouldn't support that change, nor do I think it would help.
S4 w/ a shield on increased my parry % from 10 (in s3) to 21% on average (this was before power, not duration was reduced). Any time I tank, I would instantly swap my shield in and receive a bigger benefit. S4 would then effectively be a useless stance.
Furthermore, the whole point is your potential dps on raids. This would generally be when you arn't tanking, and in s2 which is more dps than s4 is.
7-8% increase in melee dps I don't feel is significant enough. Maybe you do good dps compared to sk's. You also have 25% tomes completed which is causing you to make up for a lot of lost ground. I am 8% tomes ahead of jenks in tomes and his dps can certainly supersede mine.
Things plain and simply start favoring sks at the high end. They start to get high cha, they also start to get charms that increase their chance to crit spells. They get big high delay 2hers which allow them to more easily slip spells in between swings. They can more easily max their attack, when paladin thaz gear generally allowed pallies to do it easier.
Quite frankly I have given aux tanking with a 2 hander a try but the problem is if the warrior drops or an unexpected add comes it is bad to be stuck there without a shield as I can easily die in the first few seconds before I manage to swap out my two hander
Yalls conversation actually gives me an idea:
Instead of making s4 only usable w/ a 2her, s5 (if made better) could also be made to be 2her only. Thus making it even more of a tradeoff for the paladin to switch into their dps role.
So in total, s5 could:
-Decrease your tanking abilities to some degree
-Require a 2her, so paladins not only have the added difficulty of maintaining a good 2her
-Require a 2her, so when using the stance you would be a loss of ac/tanking ability due to lack of shield/ac on 1her.
-Require a 2her, so that you are at a few seconds loss when trying to become a better tank (time spent switching out 2her out for 1her/shield).
A combination of those, or maybe even all of them would force the paladin to give up a lot in order to significantly increase their dps. It would also make it grossly inefficient for the stance to be used in grouping situations, and only really on raids or situations where the paladin is not tanking. Although, keep in mind just making the stance 2her only would generally be more detrimental than losing "tanking abilities to some degree." This is because if the pally gets agro, he can always change his stance as soon as he gets agro and effectively mitigate or totally dodge any loss in tanking ability by being quick to change to a better tanking stance. However, being stuck with both changing stances and changing weapons will take more time and cause a greater loss when having to transition.