I've heard people say this has some effect on how hard it is to dodge/parry melee attacks. Is there any truth in this or is it only checked against ac and armour deflection?

>I've heard people say this has some effect on how hard it is
>to dodge/parry melee attacks. Is there any truth in this

None.

From the helpfile:

"The Aim skill denotes special training in landing blows that are timed and placed to best avoid an attacker's defenses. A skillful attacker gets more benefit from their HITROLL as they find the gaps in their opponent's armor and technique."

Is 'find the gaps in their opponents technique' here a red herring or does it refer to their armour use skill only?

Armor use.