If the man can become the form and have all of the memories of the deceased he shares the same power that the monster does. But that isnt all the smoke monster told ben, as Alex, that he was to follow John Locke and do whatever he says. Also durring "Dead is Dead" when john goes to find rope to get ben out of the hole only then does the monster appear. and even earlier in the episode when ben tries to summon the monster it wont come. John was also standing outside.