This approach is cutting off a major strategic artery. Sales & marketing can do much more than just pushing what they're given; they should be a valuable pulse on the customer, and the truest feedback you can get for an upcoming feature is whether or not someone will pay you for it.

Particularly for larger or more advanced features, it makes much more sense to run engineering partly in parallel with sales & marketing (with feedback between the two) rather than in serial. Product and innovation cycles in the company are much tighter that way, and more accurate.