Honestly? If you REALLY want to know my opinion, I think in many cases being an FS with no attack skills whatsoever is unnecessary unless your party sucks. Like I said, the whole point of an FS with absolutely no attack skills whatsoever is to fix other peoples' mistakes, so the very implication that your party needs an FS healer instead of a hybrid healer indicates that your party needs work.
HOWEVER, an FS in combination with... say, an STR healer in a difficult or long stage is a very lethal combination. Because then the STR healer can focus more on smashing stuff instead of healing. Things always seem to go to hell when I have to break off my attack to heal someone (leaving that monster I was fighting to go kill someone else). Even then, I'd still prefer that FS healer to at least have moonlight pieces because dammit, my parties don't suck by shear virtue of ME being in them! If Rofllollmao (a PvP healer that doesn't even have e-heal) and I can get a party through cobolt raid on at least two+ different occassions with no other healer, then an FS healer with no attack skills whatsoever is completely unnecessary.
Currently I'm main-ing two full-time fisher-girls. They fish. A lot.