The crit lines are expensive and add very, very little.
Of course an FvS should throw DP into their melee routine in almost all situations (unless their SP must be conserved for healing only, which is rare). But speccing it to the max - on a build that, according to the OP's requirements of high HP - will need 3-4 of each of racial and class toughness, and a lot more - may not be the right call.
Assuming 180 DPS from DP with no crit lines and no crit items, adding blue scale and maxxed crit lines adds ~30 DPS. That costs you red scale (~12 DPS on an eSOS twitcher) and 12 AP. 18 damage per second for 12 AP is pretty marginal, IMO. (And less than 18 DPS in fights where the 20 second rampup time is a significant fraction of the fight duration, which is often).
Edit: All of this forgets the guards on Red Scale, which contribute ~10 DPS against a non-fire-resistant boss that swings one time per second.