^ yes on both counts.
I needed the mages, they were definitely a "force magnifier." But, that's no mystery -- I mean, we're a troupe. How many games can you play where you can eliminate half or more of the group without changing the power level?
And, yes, they needed me. Very few of the monsters could have been managed by a level-appropriate summon.* Such a summon wouldn't have been able to scratch most of the enemies. If that hadn't been the case, I think I would not have done my job in sufficiently optimizing my character. I would have gone back to the drawing board. As it was, I had to nerf my character a bit after realizing how powerful a combo was.
It's possible that a caster dedicated to minions could have replaced me. But, that would have involved one of the casters radically changing their build to do so. I mean, it's certainly the case that a Malconvoker can replace a melee character, that's kind of the point of the build. Ditto, a gish.
It's the case that the casters ended some encounters before they had fairly begun. But, that's what it means to have a save or suck/die caster like a Beguiler along. That's no surprise, nor is it a real issue.
But, look at it this way, the Anima Mage with persistent Polymorph + Otherworldly and persistent Bite of the WereX still couldn't hold a candle to me in combat against the enemies we were fighting. He could swoop in and maybe do something without worrying about being wiped out in a round or two -- he had a high AC and plenty of defenses. And, he might have been able to even stand against these enemies in melee combat and outlast them. But, it would have been a much closer thing.
As a side note, I had a similar experience as a character in a high-level game alongside another BFC Mage and a Factotum. And, I think the players of the monk-esque character and the totemist skirmisher would say similar things in our current campaign where I play a sort of weird homebrew thingy kind of like a Beguiler.
EDIT: note that this is a counterfactual, so I can't speak with 100% certainty. It is surely possible that the Beguiler and the Anima Mage would have emerged unscathed from these encounters. They are both skilled players, good optimizers, and had potent builds. But, at no point did I ever feel useless. And, at no point did the other PCs "step aside" to make me feel better. My melee character never felt superfluous. Quite the contrary, actually, I consistently felt powerful and dangerous in combat. I was easily their equal in the things I was meant to be good at.
I can speak a little more authoritatively about the more recent game where it was me, BFC Mage, and Factotum. There, I'm pretty confident the others would have died without me. It was usually all the BFC Mage could do to lock down part of the battlefield, leaving me and the Factotum (who was sort of lock down oriented) to handle the rest. And, the characters operated well with a division of labor. My character would target spellcaster type characters b/c he was well-built against them and kill them quickly and efficiently, while her BFC was more effective against big brutes with fewer special abilities, so she would focus on locking them down.
*note the qualifier. It's possible that with sufficient optimization hijinks, like extensive use of Planar Binding, that the character could be replaced. But, you can use PB to get a Pit Fiend or something, and that's not the sort of thing that sees actual play.
