The real advantage in telling the "idea guy" to try to build it themselves comes when they realize that programming is difficult. Sure, they could master it if they were willing to put in the time and effort, but their interests center around the business: sales, investors, etc.

At this point, the idea guy has two choices: either understand that they've got to bring something more than ideas to a partnership, or stubbornly cling to the hope that their idea is enough.

The more people that realize that ideas alone have no value, the better.