Well, I use them more of a support or 'surgical strike' unit than a genuine tank. Generally I cast Heavenly Guards on turn 1 and then wait them on turn 2 and select a soft target. Their other ability buffs the Inquisitors/Paladins, so I use that sometime, and their high hp means that you can take some damage with them. Plus they provide a morale bonus for human units, so for me it's win/win/win.
The only downside is the morale problem with Dragons.
