Product managers sometimes get dragged into sales calls even though sales is not among their core responsibilities. Product managers, after all, are experts on the market, understand the product, help devise the sales process, and formulate the key messages. What better person to take on a sales call?
Sales people and product managers should work closely together so that the sales people are able to operate independently with customers. A product manager has not really fulfilled his full responsibilities if he hasn't communicated his understanding of the market with the rest of the organization.
One exception to this rule is that it may be beneficial for product managers to "test" the sales process and messaging by personally applying them with real customers.