All heroes are viable on attack if you're willing to play them creatively. Offensive torb requires more effort but his turrets can be a wonderful distraction and, placed in just the right spot, are good for helping your team push. Think of putting it in more narrow, strategic angles rather than giving it full coverage. Just somewhere with decent line of sight behind the bulk of your team. His regular gun does a lot of damage and has range. It's nearly as good at poking as Hanzo. And the armor kits give a pretty decent boost to your team. Torb's ult, if you have a good turret up, is also good for clearing space on a point.