Of course it needs its damage boost in PvE. You are forgetting that in this game tanks have far less extra threat than some other games. In order to hold aggro tanks actually need to be able to deal damage - and to get FL near the numbers you are proposing assassin needs to stack loads of power (my pvp sin has crit/surge build and FL, even with all the stuff, ticks for max 1500 on undergeared people and 1000-1200 on geared persons).
For the record, mara/sent damage is very fine indeed. Maybe they have too much defense for a dps class, but their damage is fine.