you can have several reasons :
- first, some players chose to defend schneider by following him more than defending consoles. sometimes it could be tricky to fight him alone, trying to put aleph on him while 5 grunts are unloading there shotguns on you.
- most chose to reduce monsters, running a little everywhere. less monsters, less chance for everyone to die (including schneider) and as soon as you see an engeneer somewhere, you go for the kill before it makes troubles.
- some consoles are pretty far away from each others and there is often one or two doors for incoming monsters near them. and maybe you don't want to be alone to face them all if a big pack comes in.
- as you don't necessarily want to be isolated next to monster entrance to face an antag.
- staying relatively close to each others give everyone the possibility to care for anyone seems in trouble.
- depending on your character and the game difficulty, you can't face more than two or three grunts at the same time without having great chance to die. some characters are not really made to defend position.
so you can tell me, "ok but if you know it is this mission, take one that CAN defend a position"
indeed. but you need to have one. characters are expensive, people are not all paying to have gold, so maybe their pool of character don't allow that, maybe they are used to play specific characters not suitable to defense, but they like to play them anyway, maybe they noticed no one in the team had weapons allowing to shoot the flower on the next stage, etc, etc...
- and finally, some just play, running around, killing things, "brainlessly", being happy.
it's still pretty usefull until there is something really more important to do, and they should do it because they are the best for it and no-one can replace them at that moment to do it, and well... they don't...
here goes the world =)
but to conclude, imo, one player defending each console is not necessarily the best thing to do depending on the team, situation, etc.