If you're a business, and you're advertising on Google, but not on Bing and not on Yahoo, you're probably being pretty stupid - unless the revenue per dollar spent is higher for Google than for Bing and Yahoo. If you can demonstrate that it is, then you might have a case.

I only said "might", because the question isn't whether Google is more effective per dollar. The question is whether advertising on Bing generates more revenue than it costs. If it does, then businesses would be stupid not to also advertize on Bing.