The only situation I see in using a PLD to remove a WHM from the equation, is to run BRD/Thaum. Or some kind of secondary healer/DD/buffer.
As others have pointed out. It's not like PLD can solely heal itself, so the other members are gonna need some heals. But at least it gives the opportunity for more close range melee, to join in on the fight. Instead of Ranged Burn everything.
Not trying to be a douche, but let me fix that for you Dzian
A 12 minute fight is 33% longer than an 8 minute fight. I suppose in some sense you could say that makes the 8 minute one 33% more safe, although it wouldnt be entirely accurate but there is some logic in it.