I think Roac is looking more at the long term situation which neither Dain or Thorin are. Thorin wants to keep his stuff, no matter what, and probably figures (without any specifics, how this would work) that with the support of more dwarves, they can figure out how to survive with unfriendly neighbors. As a matter of fact, based on the dwarves behavior in the Hobbit, they probably usually have to live with unfriendly neighbors! Dain is rushing in to help in a crises and doesn't have many details of what the situation is.

I bet the dwarves didn't help Thorin because they didn't think he was likely to succeed.

Roac knows that if Thorin knows that reinforcements are imminent, there is no way, he will keep his agreement to trade for the Arkenstone. The knowledge that Dain is on the way just about guarentees a battle.

"...men's hearts are not often as bad as their acts, and very seldom as bad as their words." Tolkien, letter #250.