Feature Evangelist vs. Product Evangelist

I've seen a lot of start up companies over the last few years that came out for the gate with really exciting, clever, and popular products only to see them stagnate. Lately I've given some thought to the cause of this phenomenon. One problem I've seen is when the product team focuses too hard on a specific feature request from a specific client. Typically, this is a big, well known, or VIP client with an internal 'friend' or evangelist for the start-up's product.

The scenario goes like this...
The Evangelist calls up the Product Manager and says that there's some growing traction for the product among senior management. A deal is likely, but that the start-up needs to make some changes to the feature set to make the product meet all of the features in the Request for Proposal that the potential customer has been sending out. The Product Manager, getting pressure from Sales and Marketing, is eager to help land this great account. One of the worst parts is that at some point, and probably unbeknown to the Product Manager, the sales person has promised a deal to the client that has all but precluded any profitability on the account. So now the start-up is in the situation of having to spend time and resources on a new feature for a product so that they can sell this product at a loss.

This happens more that you'd think, and it's too bad. The solution is not to ignore the customer (that comes later, if you're a Geoffrey Moore fan) so much as to be aware and on the look out of the customer or client that is too focused on a few features. Ask yourself if the client really is a strong supporter of your product, or are they simply using your product to advocate for a pet feature that does not represent an overall product improvement