The 2 are not related, I've never questioned why our tier bonus for tanking is damage and not healing/avoidance of some sort. I can say that building to the 4 piece set is golden with the IBF duration being increased.
Doesn't this fall in line with other 2 pc sets, I mean normaly the first part of the set is only icing and not the cake, to enjoy both you have to invest in all 4 pieces. The Pally 2 piece increases their hit rating, and the 4 piece increases their dodge.
I guess it's in line with others though I haven't looked at all of them.