If this were ever entirely true, it is certainly not now. With increasing competition, not only must salespeople sell their companies' wares; they must do a lot more besides.
As Fred Wiersema, a US management consultant, points out in a new book, "customers continue to raise the level of their requirements, but their range extends beyond best price and best product". They want superior results - and that leads to the title of his book, Customer Intimacy.
Mr Wiersema, co-author of The Discipline of Market Leaders, argues that the shift towards customer intimacy amounts to the third stage of companies' quest for value creation (after the drive for continuous improvement in quality and the radical redesign of business processes).
As he explains, customer-intimate companies "seek solutions that exceed immediate needs and demands", which may mean giving different levels of service to different types of customer.
The point is developed by another US consultant, Wayne Burkan, whose book, Wide Angle Vision, suggests that companies can beat their competition by "focusing on fringe competitors, lost customers and rogue employees".
He quotes Livio DeSimone, chief executive of 3M, as saying he wants his salespeople to look outside the organisation and interact frequently with the customer. As Mr DeSimone explains: "the most interesting products are the ones that people need but can't articulate that they need."
Many of 3M's famous innovations - from masking-tape, to a means of mopping up fluids during surgery - have been a response to customers' needs. Scientists in the research and development laboratories have known about the needs from salespeople who have been encouraged to report back.
But no one ever asked Sony for a product like the Walkman. Nor will market research always show the way.
Mr Burkan suggests "walking around on the edge", as unfulfilled potential lies outside the mainstream.
It is not, however, easy to get sales forces to behave in this way. For example, increasing centralisation of drugs purchases has meant that sales teams who once built up relationships with individual doctors now have to deal with impersonal organisations, which will be continually seeking fresh forms of delightn