I would not suggest/allow heroes to equip armor during an encounter.
By the rules, they are allowed to do this, so I don't think this should be barred. This is also a very useful strategy when playing with someone like Nanok of the Blade. He can equip the armor on rounds where he needs more defense, and take it off when he wants to use his heroic ability.
That's the problem. I try to play "realistic". I frequently ignore rules that don't fit in my opinions. But you should agree, that put an armor is not an easy job. Not to say that if the hero is putting the armor and then casting a spell on the same turn. It's too much for me.