It really isn't...
When did everyone decide that un-carapaced skulks are worthless? I rarely take carapace as a skulk. I find regen much more versatile, and sometimes just don't worry about taking upgrades, as I can do plenty of damage most of the time with a plain vanilla skulk. (Of course, I really like getting sensory or movement up first too, then it gets fun!)
As others have said, it's all about hiding, surprising, in other words: SKULKING around. Sure, you're in trouble if you charge at a marine from 10 meters without carapace, but guess what? You're in only slightly less trouble with carapace.
My theory is that for many people, carapace only allows people to continue to play sloppily. It gives you more margin for error and allows you to learn, when you're first starting, but it also allows you to keep playing with bad habits, instead of discarding them.
Think about it. If you are playing with the keyboard only, and no mouse or mouselook for you, then carapace is going to help you live longer, but you would do much better by learning to use a mouse.
When I do use carapace, I notice a difference because I can often get 3 or 4 marines, instead of 1 to 3. This is because I am plenty good without it, largely due to my tactics, and I still use these tactics with carapace.
To sum it up, use carapace to extend your already lethal skulk life, not as a crutch to allow you to keep playing poorly...