It's not a prevention as TCP is recommended in most of the cases of bacterial skin infection (and usually works) where a doctor may prefer to prescribe. I think the first answer is probably closer in that it may only work in less advanced cases and stop them getting worse, but not have the power of the penicillin based ones. But I didn't think many germs would have survived neat TCP either.
Cheeky chops is definitely right, an antibiotic has the power to target certain bacteria, you get some that target only one group and wide range ones, that will kill many groups of bacteria. An antiseptic starves the bacteria, and inhibits it from developing and may cause it to die with regular applications. Antiseptics are okay for acute conditions but for chronic conditions they wouldnt be advisable as they are irritating.