Full Immortal/Defence is viable. Your actual dps is lower, but your threat modifiers on your abilities are higher to make up for it.
The actual threat generation and mitigation are lower as well. It's not just "lower damage, same threat and everything else"; the hybrid is generally considered superior mainly because it has generally been shown to *be* superior, as long as you've got the chops to play it.
Of course, all that said, full Defense *is* fully viable: you'll take *slightly* more damage and generate a fair bit less threat (which you can solve by taunt fluffing like most Guardians are forced to do anyway), but you'll still be fine.