Google is a big corporate machine, so changes that they either want to do or need to do move slowly. The ROM/kernel developers can deal with bug fixes (things like, but not limited to wifi connectivity, wakelock issues, UI issues, overall battery life, and on and on) faster.
As far as ROM vs kernels go, I've read it described as the kernel is like the engine in your car. It's what makes it go and determines how quickly it does it. The ROM is the body...your UI that tells the engine how to run.
As far as backing up goes, nothing is a more complete backup than a nandroid backup, and the only way to do one is by being rooted. Nandroid backups allow you to switch between ROMs in a matter of a few minutes. Right now, your Nexus is new, so it's both fun and satisfying. 6 months from now, you might start getting board with it. If you start playing around with ROMs, it's kind of like getting a whole new device. This holds you over until the next Nexus is released.
Whether you decide to root or not, you should at least unlock your bootloader as soon as possible (because you loose everything that one and only time). If you never decide to root, that's fine. But, if you do decide to root after you've had the phone for months, you'll loose all that stuff you've accumulated on the device when you unlock it. It's best done in the early stages of having the phone.