What you have to remember is that balancing between PVE and PVP have different things to be considered. Whilst PVE is being balanced against each other to fight a static encounter, PVP have to be balanced not only between each other, but the dynamic environment of a human on the other side.
This is why SE balances in each role. All the tanks are similar enough that you can choose any and be fine. Start adding synergy between 2 tanks, and you would have to add it for all different compositions, otherwise, you start having imbalances. That is 6 different combos for the tanks, not much, but it is still something to think about. Then you have the fact the synergies will not apply in dungeons, which SE do balance around.
If anyone can give examples of potential synergies, I am willing to look at it as there might be something I have overlooked.