you know, D&D *is* a collaborative experience, which is something DM's can forget occasionally.
If a player wants to say something like "hey, could there be a trap here? I think it would really add to the dungeon...", sometimes, I think the GM should step back and say "why not?"
In the mentioned case, I think the GM should appreciate something like this. The player, paying attention to the scenario, has decided to take a gamble. As the GM, I'd say something like:
"Hell, I'll one up you. You roll a fifteen or higher, and they get the bonus. You roll a 20, and I'll double it."
Anything that encourages the player to take up the GM hat, even for a moment, is something that should be cherished; after all, it means that the player is involved in your campaign world.
My two cents.