Need? Probably not...but if you want your kit lens to take out in place of your better glass when certain conditions apply, can't hurt? I wouldn't miss my kit lens if I dropped it, as much as my Tamron 17-50.
But then again, if I wanted to make the best photos possible and the reward outweighed the risk...I'd leave the cheap glass at home.
You don't "need" the 18-55 but I would keep it simply because they're worth so little if you sell them that they're probably worth more to you to keep around as a backup.
Hmm. This means I need a Bigma (150-500 HSM) as a backup for my 55-300.
I think you're stretching the concept.
To get back to the original question, I can't imagine a time where I'd put the 18-55 on my camera when I have a better zoom in the bag. My idea of a backup is that the primary lens is whacked and I have no choice.
i sold my 18-55 and never looked back. but i got so little for it keeping it might have been an option. not so much for backup, but in situations where you want a lighter setup.. camping, long walks etc.