[sector24]

Never be afraid to fill all of an important unit's slots. However, most armor increases encumberance, which increases fatigue. So if your gorgon flies in there and everyone gets petrified attacking her, you definitely want a shield, armor, etc.

For a spellcaster, you want to mind your encumberance because it adds fatigue to every spell cast. So in heavy armor you essentially knock yourself unconscious. However, there are plenty of items designed for mages to wear that offer protection with little or no encumberance penalty. Some items even grant reinvigoration, which removes fatigue every turn. Basically the opposite of encumberance.