I use guard mode religiously as the Romans but then I tend to fight very defensively and wait for decisive moments in the battle to attack directly and in force. The major benefit to fighting in Guard Mode as I see it is that if you have not ordered an attack your units will concede ground under the mass of the opponent while maintaining their formation. This is exceptionally noticable when facing a cavalry charge with any kind of non spear melee unit that has a reasonably tight formation as standard. The formation will almost never break if sufficiently wide enough to prevent envelopment and will generally halt most charges completely in their tracks with limited casualties by conceding ground under the mass of the enemy unit.

In Guard Mode the Flanks are very important, but with secure flanks Units set to Guard Mode On will survive much longer under sustained pressure and allow for decisive actions to be completed. If you intend to absorb any kind of a charge from the opponent then I find it rather perplexing to not use Guard Mode and have you units stationary and without orders facing the opponent. Attempting to resist the charge will result in your formation being broken or infiltrated with significantly FASTER casualties. The casualty rate may be the same if left unattended, but the decreased rate of casualties will allow you to bring up support units in flanking attacks.

Indeed if you have for example three units of Milnaht set in Guard Mode and alongside each other then issuing an attack order with the two flanking units after the centre one has resisted the charge will result in complete butchery of the opponent with minimum possible losses because at no point has any of the three units or any of the men in each of the units exposed their right or left side to attack. The enemy will have to fight you frontally at every single point of combat.

Obviously this example requires a 3 to 1 advantage, but it is the ideal and should be considered as the ambition wherever speed of your victory in the local area of the battlefield is not a concern, or indeed where speed of your defeat is a vital concern. If you happen to be up against some kind of Elite Unit like Gaesatae and have managed to leave yourself with only 3 Units of Hastati to prevent them rolling up your line, then Guard Mode is a no brainer for the centre unit, but it is also a no brainer for the two flanking units as they can attack the Gaesatae at weaker points of their defence while forcing the Gaesatae to fight through their strongest point of defence.

Guard Mode On prevents units at the centre of a formation from being attacked from the sides. Guard Mode Off gives no such prevention and allows any unit in a formation to potentially be attacked from defensively weak angles, irrespective of the orientation of the formation.