Iss armor type

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