Recently I was talking with someone and he asks me this funny question.
What is the difference between a Field Applications Engineer and a Sales executive apart from the fact that FAE has technical responsibilties? In a broad sense, both belong to the sales organization and are part of the strategy process to win a customer. But one of the fundamental difference I think is : l the sales manager sells to the customer by saying there is donut and the job of the FAE is to say there is a hole in the center of the donut 
. I completely acknowledge that Sales is no joke and this is how it works in this world.
If we dont tell about the hole, then 80% of the time, the FAE spends fishing up the customers who fall in that hole. It is a lot support intensive task and the FAE can better spend that time else where and generate the revenue for the company.
In general the FAE is concerned with making sure that the user and technical evaluation proceeds as smoothly as possible and the Sales person is working the buyer, legal, and purchasing issues. Normally the sales person is accountable for closing the business (and will be fired much more quickly than an FAE for failure to do so). Every new technology has issues and it can seem that the sales person is minimizing them, but most experienced sales people understand that a satisfied customer can offer a reference that makes future sales, especially of a new product, much easier.