As a know iss have mastery in all armor types, usually everyone goes wit heavy, myself included, i seen iss in robes n wit staff, only thing i could think of was the extra mp u would get, idk if that would effect the landing rate of skills like seal of limit n blockade, song of silence, shed some light if u can, i know everyone got their own style of play, just wonderin if it benefits anything using robes, i go wit heavy for survival reasons for sieges n pvp
No, there's no reason to go for robes as iss. Obviously, you can see some Iss'es using robes during oly if they r facing feohs/healers, since they get extra mdef/mp and casting speed. Other than that, it's awful for pve play.
Your main goal should be focused on heavy armor (better defensive type), and in some occasions, like doing instanced raids like Istina/Octa/kartia, you can swap to a light set of the same grade, if you have the resources. Why? Well, you'll get some extra mp regen (really good), some extra speed and a bit more of power in skill usage. But this is only plausible if you can manage to get away from massive dmg spikes.