Poison medics are one of the most annoying classes in the game to come up against if you haven't got a team of competent healers. Against a lone recon or robo without a med crate, or large groups of people without a medic that can pop healing wave or grenade (admittedly rare, but still) they can inflict stupid amounts of DoT - somewhere about a 10th of my health each second on my recon. I'm probably killed by them the second most, the other being roaming tanks with a pocket medic.
That said, it's much easier to be a good poison medic if you know how to be a succesful healer first.
Snipercons don't really steal points in my experience - they're more of a point denial class. Bombercons and Melee/SMGcons are more suited to that to be honest, although there isn't any rule to say that only a certain class can capture the point, or in fact do anything. Something people seem to forget when shouting at the recon for not destroying the turret they so happily stand in front of.