I never take Guard myself.  If a hero comes with Guard, I'll use it if he has nothing better to do, but Guard as implemented now is nearly useless.

I would favor the suggestion of making the effect stay until the caster's next turn.  Also grow the effect area beyond the arc of 3.

I agree that the Guard ability wears off too soon. I would like to to last until the Defender's next turn, rather than the units it effects.

OR

Have the ability effect the tiles instead of the units. It would "fortify" the tiles, giving any friendly unit in them a defense bonus equal to the Defender, and prevent enemy units from entering those tiles.

It applies Guard mode on the applied unit, so the flat armor increase + guard = 9 increase.

it puts the unit in defense mode, so the unit gets a flat + 5 defense plus whatever bonus it has for improved defense mode (defensive trait +10 def, shields add +5 to +20 depending on type of shield) additionally, the guard ability adds the base defense of the caster to the buffed unit, which can be pretty high considering defender champs can wear plate armor. timing is important, though (probably too important), since it only lasts until the buffed unit acts. so if you buff a unit that is next in the initiative list, it's pretty much useless.

one reason why this ability is so weak is that the caster isn't affected, so he loses his own defense stance when casting the ability.

If it's an ability that can be spammed (namely, cast indefinitelly without cooldowns), why not just make its duration infinite and only end when the guarded unit moves away from the guardian or when the guardian decides to take an action that ends its defensive stance?