There isn't any guard mode bonus.
I've recently noted that the way "guard mode" work depends only from radius attribute.
The radius standard for all units is 0.4 (space single man takes in the field), so diameter 0.8.
Now, thinking about a unit in guard mode: the inter-spacing between two men... it would be less than 0.8... especially when the unit is under enemy pressure.
If you switch off guard mode , you can see that your men take more distance between them(during enemy attack).
Then, when men are open in this way, they are more weak in melee.
Isn't it right?