Even if you say that FH is too strong right now, I don't see how anyone could make a point of making a legendary skill healing you for a few hundreds HPS in a meta where just about all the viable specs sustain 20k-30k+ DPS with burst of 50k to 100k+ DPS.
Spend a legendary point to lower the overall damage input from 100k to 99600? Seems a sound legendary point investment.
I guess it would be useful against people dealing 2k DPS, given that it would block 20% of their damage, makes it on par with the legendary skills that increase your damage by 20%.
a 10%-effective FH (following the pvp rules) would be OK if you fight level 55-60 players. Not excellent or anything. Just OK.