Rebooting certainly "solves" many things, and is sometimes the simple solution. However, many geeks - Linux users in particular - prefer to find out what's wrong, and how to fix it, seeing rebooting as "taking the easy way out". That is, it works, but you don't know why it works, and if it ever comes to a situation in which rebooting isn't a viable option, then you're stuck. For some, it's also a matter of /wanting/ to understand.
Also, for that matter, rebooting the phone while on the go is hard on the battery, and thus restarting only X would be preferable.