On general you are right. Most pirates cant replace archers. But rachel is slightly different because of her active which turns her into a better long ranged archer.
Put it this way, there are 3 plat SS tier units in DMM that are useful in most situations according to the hardcore discord aigis members.
2 of them are not in nutaku yet (Saria-healer and Totone-merchant), the last one is rachel.
Ok, part of the reason for that is her SAW, but she still the top tier unit when you crunch the numbers.