You point is analogous to the idea of sales and marketing in start-up culture. Essentially how do you prove that this multimillion dollar deal was the result of the person being "fresh" rather than the product itself etc. As far as I know if you're being flown out as a sales rep already, you have to be an utter asshole for the customer to change his mind. The product/engineering teams are the ones that actually have to impress them. In all honesty when a company through a rep wine and dines a client, the rep wine and dines themselves.

Conversation like this are not unknown:

"We need you to fly to <FarFarAway>, urgently!"

"Business class?"

"No"

"OK I'm not going then"

"Business class it is then!"

On an overnight flight? Huge difference in performance, at least if you're me. I can't sleep at all in economy, which if you're flying 20+ hours is a really big problem.

I've never tried first class, but I'm sure it'd be much easier to sleep up there, what with the lie-flat beds and all.