IMHO, none of them are useless, since all of them have their unique uses - in one way or another.

For one, Freya isn't useless. Although she needed some serious leveling up to do when you are forced to use her in Disc 3, she's fine.

Quina annoys me sometimes, though. However, his/her ability to use powerful Blue Magic makes him/her useful. (Haha, Limit Glove)

Although many people state that "one summoner/white mage" is "enough", I find having Eiko in my party very convenient... especially in Disc 3, when Dagger had some problems with her concentration (or something). Plus, isn't it very useful to have one white mage summon an eidolon to deal damage to an enemy, and another white mage to heal the party? That's just me, though. XD

Amarant, on the other hand, had little use to me on the first few events. However, as the story progressed, I found him to be quite useful with his "Flair" command.

Beatrix isn't useless. Period. Wish that we could've had the option of keeping her in the party, though.

Marcus/Cinna/Blank aren't useless either. I would've been dead meat in the Evil Forest if it weren't for Blank. Same goes for Marcus, who helped alot in Disc 2. However, Cinna, IMO, is "sorta" useless (which is why I cast my vote on Marcus/Cinna/Blank), but that's just me.