“We’re doing X to do Y” is a fine pitch, as long as X would be a viable venture scale business on its own. If it’s not, you’re asking an investor to bet on one risky venture that merely enables *another* risky venture. Why do that when they’ve got access to any number of deals that promise huge returns on their first iterations.
Often, founders would be better off letting VCs fill in the gap of Y themselves, which they will if they’re a) smart, and b) excited about concept X. Check out this story about how to pitch Jeff Bezos, the same principle applies: http://techcrunch.com/2011/10/22/how-to-pitch-jeff-bezos/