Soldiers don't follow orders.

[Thaatu]

I'm too lazy to check if this has been mentioned already, but switching guard mode on prevents the enemy from "snagging" your units into melee. In fact there's an exploit, which some of you are probably (accidentally or not) using, in which units that are defending against an attack with guard mode on suffer no exhaustion.

[Xehh II]

Units in guard mode do suffer exhaustion, it's just very, very slow.

[pezhetairoi]

I notice some units die faster on guard mode, though. Notably, my Neitos and other sword infantry.

[Xehh II]

I'd expect that guard mode would best best for Def. units, swordsmen aren't really that defensive.