While it's true that MA doesn't have much of a penalty compared to HA, and it does offer over twice (usually) the protection of LA, it also has a disadvantage. LA always has +1 dodge, and in some cases +2; HA always has +1 parry and goes up to +3; MA should always have 0 in both, and in case of a well crafted set, up to +1 dodge and +2 parry. MA on the other hand has an inherent HP bonus (equal to half the quality of the armor), which results to double the HP gained compared to light, and can also be used for fight skills (with parry mod ranging from 0 to +2).
So to sum it up:
LA: For when you're in a big group with a dedicated tank, able to kill the mobs you hunt before they reach you or when the mob doesn't have a physical attack (psykopla/jubla/stinga). For focus gear this is the only viable option.
MA: For tanking mages who need some extra protection and HP or for light fighters (fighters who rely on their dodge/parry to reduce damage taken rather than their armor). The "good for all, master of none" armor.
HA: For tanks only. Nuff said.
MA looks a lot better than LA though
