Defender is a tank/support mostly, you have to play them as such.
A defender will not be used to win battles themselves but they will save your other hero's butts and allow your regular unit to stay on the battlefield longer.
Here are some examples of how I like to use a defender:
I really like the Guard ability. I just put a row of tree units in front of the defender and cast guard on all 3. On some battles it can make a huge difference by allowing a few crap units to tank lots of damage.
I will also use them to heal other units or save them by switching place with them.
In general you gain by having a defender with lots of armor and high initiative. So that means equipping him with sword/dagger + shield and mounted on warg.
Edit: Also, don't you hate it when you fight a bunch of archers and they decide to all attack your weak mage at the back?
Brave defender can stay by the side of the target and cast heal and guard.