How much damage could one do to an unrooted device? I'm trying to determine if it's necessary to backup my non-rooted tablet. I plan on doing a lot of messing around but most back up utilities require root and the root exploit hasn't been found for my device yet. Is it worth it to do a quazi "full backup of non-rooted device"? 

So let me clarify. Am I right in assuming that if the device in non-rooted then the system cannot be damaged by malware or misconfigured apps? If I'm installing just installing apps is there any chance that I could damage my non-rooted device beyond trivial repair (i.e. not having to take it into a shop)?
From within the Android system itself, all you risk on the great majority of devices is losing data such as media, apps, logs and configurations. Anyhow, if one manages to damage it more than that, it is usually repairable through a factory reset.

Whether or not your device is rooted, you can always do it harm by flashing bad bootloaders, recoveries/recovery images, inadequate update.zip files and a few other things, so always watch out for that.

I strongly recommend that you do a backup anyway -- it's a fairly simple process and it's worth it just to stay on the safe side.