Michael Twiss was poor last season, of that there is no doubt. However, the concensus also seems to be that he's gone away in the summer, lost a bit of weight and is now trying a lot harder. He'll score goals but probably not as many as Damian Reeves will.
For the record, I don't think that Twiss will ever be able to live up to the level of expectation that we had when he first signed. I reckon a lot of us (myself included) thought we were getting another Colin Little who would bag 20-30 goals a season. In retrospect he's been nothing like the kind of player Colin was and is never going to be either. However, if his improved efforts lead to Damian (or Astley for that matter) scoring more goals then his presence in the team will be completely justified. If someone better comes along and displaces him then so be it, but I think he deserves credit for his attitude and application so far this season.