Because even in most fights that require two tanks at any given time it's just one of them tanking the boss, while the other is dpsing?
As for sam as long as potencies are unchanged it will keep the gap size relative to other dps even as ilv increases, though at the moment some of the other dps jobs are quite lacking so I think we can expect some of the lower dps jobs being brought up closer to sam, especially the ones with less dps utilities like blm.
