I never really thought about the other reasons, other than camouflage. Very interesting.
Another question: why is it (on the newbie server) that you have to wait for a while before you can build? And why does the map start with no armoury sometime?
Thanks everybody for this thread and actually this entire forum section. I've been reading it for a few days now. Tonight I got to put it to good use, remapped my binds then practised for a few hours on the newbie training server.
It ended at the wrong end of a rant stampede most times because we played against some alien players who weren't exactly newbies, so we were forced to hole up in out base most of the time. So it was mostly an exercise on surviving under siege.

But it was good, I think we put up a good fight and I got to see and try a lot of things I've read about in here. I'll try to summarize:
* Don't feed the aliens. Don't respond to taunts and go out alone, especially when you're not armored. If they enjoy staying just out of range and teasing you, let them, if you know you can't survive out there.
* Watch the lines of fire, both of your mates and of the turrets. Also try not to block their back or their sides, and cover each other when advancing.
* Exercise self-control with the trigger. You don't HAVE to empty a clip in one go. If a mate comes into the line of fire you can stop. If you only get line of sight on an alien for a second it's better to squeeze a couple rounds and hit him once then shoot 30 rounds and hit him once. As a newb you go frantic sometimes and start firing non-stop when the aliens are around, but you gotta keep a clear mind and use it.
* It IS possible to outmaneuver goons and rants. It's harder with dretches, and they're all so damn fast, but it can be done. I know, what a suprise. If you play on regular servers as a newb it's all like 'put your head out--bam you're dead' so you get discouraged.
* Camping in your own base will not pay off if you're not doing it right. Gotta place your turrets properly so they can cover each other and be hard to reach, gotta place yourself properly so you can cover each other and the turrets and get covered by them, gotta cover the builder when he needs to repair or replace turrets.
* Communicate. We lost a lot of times because we didn't keep hitting their base when they were down to one egg, or because we didn't follow a wounded goon or rant running away (or followed him just one guy and ended up as food), or weren't clear on who should build, didn't notice when one had to get back to heal and get ammo etc. I advise getting binds for the most common situations.
* Use your head. It's so much more fun if you keep a clear head and use it. You start seeing you can use even the smallest pieces of environment against enemies, that you can relax right under their noses if you're calm and they're not, that you can find interesting new ways of doing something.
* I'm afraid I still don't see the magic of the shotgun.

I'm thinking it's a specialized weapon that should be mastered. For the newb, the machine-gun type of weapons (rifle, pulse, chaingun) will work best (well, considering each of their quirks), as long as you don't get trigger happy.
* Practice AND read about it. It's a great combo. I actually learned a lot more (in just one evening) then I wrote here, but it's a long post already.