And with a good team, you don't even need the turret on hard difficulty. :/How...how many ammo crates do you carry?
On Landing Bay? None. So long as you don't take the pump-action shotgun or tesla cannon, you should be able to hold them off at the choke point.
What's wrong with taking a sentry turret on Landing Bay? On Hard and especially Insane you really need some sort of fortification if you wish to survive the last portion. Besides, the game actually encourages you to use a sentry turret at that point.
There's a sentry turret in the last welded room, so you're wasting inventory space. And doing Landing Bay without welding behind you is silly. It's like shooting yourself in the foot before running a marathon.
And with a good team, you don't even need the turret on hard difficulty. :/
Welding in Landing Bay is unnecessary. You are wasting the only secondary equipment slot you have. And if you don't want to waste that slot and and still get that turret, you'll need to make 3 trips instead of one through the in between rooms to get a welder and keep what secondary equipment you wanted. Then you'll have to make again 3 trips instead of one between the room where you wanted the turret from and the final part of the level. All those trips are very very annoying, especially on Insane where you don't want to be anywhere near an alien. Instead of wasting all that time, it's better to bring your own sentry and be done with it. It's not like the anyone but the Medic and some guy bringing ammo have anything better to bring along.
To me good play is about minimizing mistakes, if that involves bringing a damn turret along (and if my experiences in Insane tell me anything, make that two turrets) so be it.
Running back and forth between areas is suicidal (though not necessarily fatal), on any difficulty. The best strategy is to move quickly, and seal off the passages behind you. In areas where that isn't possible, it's still best to move quickly and get out of the spawn-trigger areas (which is why going up the middle of the bridge level is silly, you hit all
of the triggers).
The welder is
unnecessary on Landing Bay (it's a 'must have' on Cargo Bay and the bridge level, though), but it makes the zerg rushes a lot easier to deal with.
The Technician adds his engineering bonus on to welding tasks done by other players.