The only thing I would add to this is that the skill set of being able to work in a CLI-only environment(vi editing, for example) would wane if you *never* used it. Things go wrong, sometimes you're stuck in a technical situation where tunneling X isn't going to happen, security issues - and being sharp working that way will save you a lot of time.

Depends on what you're doing, too. While GUI is great for lots of end user things, I've found sysadm skill sets aren't improved by it at all, and frequently weaken. You never want to be helpless without a GUI if you're a sysadm.