I disagree somewhat--the conductor may not strictly be needed for the performance (with enough rehearsal, of course), but it's likely to be much better with them than without. For one, keeping a steady tempo by consensus is pretty hard when you're trying to listen for it from your fellow musicians while simultaneously attending to your own pitch & timbre. (Not to mention accelerando and ritardando--changing tempo in sync is nigh impossible without a benevolent dictator in front of you!)

Also it's very easy for an individual musician to forget that oh, this is the THIRD repetition of this really loud phrase, and this time we're supposed to move on to the really quiet part, and seeing the wildly gesticulating figure in your periphery suddenly shrink down smaller is a good reminder that things are changing to the quiet part.

And as a former percussion player who had to wait for sometimes hundreds of measures of rest before coming in precisely on time with, say, an extremely important bass drum hit, it's extremely reassuring to get eye contact and a finger-point from the conductor to verify that you're not about to ruin the entire performance by coming in four measures early just because you lost count and were therefore trying to base your entrance on the repetitive phrasing of the piece. Not that this ever happened to me...