I'm not sure that many firms (if any) would specialize in hosting sub domains only (why would they?), so I would imagine you'd be trying to sell to an ordinary host who already have a developed site, and also offer sub domain hosting. Admittedly a few hosts used to specialize in sub-domains only, with banner ads/popups to cover costs, though they all appear to be out of business now.
So I'm sure that is has much appeal myself. What would a regular host do with the domain? I don't see how it could be used to generate traffic.