Casting focus cap is 12% with gm inscription providing an additional 5% overcap. CF is cool, but not something you can necessarily rely on for getting heals in, even at 17%. If you're using a spellbook w/out anat/eval or wrest, throw on a mage weapon when things get hairy first. You can go back to the slayer/sdi spellbook when it's time to stack damage and you're not so focused on surviving. If you still can't get heals in due to enemy casters, poison, etc then hit Prot until you're safe again.
To me the worst thing about prot is the FC delay, which will absolutely kill your dps on a caster. It also makes GHeals take an age to cast. Lord GOD is right in that you don't want to run it all the time, as if you become reliant on that, you're basically forgetting how to play your class. It is very good for getting that invis in, though, when shtt really hits the fan!