It is true that field sales gives a real time experience of the market and the ups and downs of the same, but it is not the necessary criteria for becoming a good sales guy, if if you have that experience with you, then it is like an additional benefit cause you know how to survive when you are provided with a market to map and pin out the prospects from it to convert them as your clients.
More over it also helps in building a strong relationship with people.. but due to this we cannot say that those who haven't done that don't know anything about sales.. It is a legendary thing according to me to convince a client, to introduce him about the services, to show him the benefits and make him believe in you.. WITHOUT EVEN MEETING PERSONALLY..

CHEERS