For a Holy Mage, an extended (non-Pikes) wall is at least 4 sbp's and will cost at least 11 mana (but QC). Another cleric is 1 sbp and 5 mana (but FC). There might be cases where it's worth doing - e.g. if you're sure that 1-2 more clerics would be destroyed quicker than a wall for some reason, or you can't afford to FC but have the mana for a QC extended wall - but usually you're probably better off spreading a few clerics around the arena.