A couple of counter-arguments.
1) You don't say no to Siakam because of Jermi freak'n Grant. If they turn out to be a bad fit - you keep Siakam & trade Grant. ALWAYS take the better player!
2) I don't believe Grant cant guard the SF position. Some loud posters keep reiterating that - without any arguments/reasoning to back it up. IMO, Grant can guard the SF just as well (if not better) than Ant, Sharpe, Little, Knox, Watford, Walker, Reddish, or Winslow. While he may not be a 'lock down' defender at the 3 - he's sure a hell of a lot better than anyone else on the roster.
3) Rebounding is one of the most important aspects of playing good defense. You can't be a good defensive team without it.