They are actually excellent in defense. They have p/d/sb, self healing, dam redux, and a nifty song that makes many skills/attacks simply dissapear. IMO they are among the best tanks in the game. They can tank almost indefinately where other classes would need rest. The only real problem is they don't know a few of the rarer weapons, making them bait for certain attacks.
If one went outlander they could chameleon and hide from quite a few mobs, making rest very easy.
Group support is also important for exploration, eventually you're going to need an invoker/healer/etc. to help you.
They can also use staves/scrolls which helps a hell of a lot.
Not much they can't do if played correctly.