It confuses input with output. If I make 10 calls to a customer service department in two weeks, it boosts its activity rate, but it means that the firm is not solving my problem and I'm probably badmouthing it to everyone I meet. Better would be one longer call that solves my problem and turns me into a happy customer who recommends the service to friends.
It can promote stupid behaviour. I recall a clever Italian medical textbook salesman, Santiago. He concentrated his efforts on a handful of top professors, knowing that by selling to the opinion-formers, sales to their less august colleagues would automatically follow. It was lucrative and low-stress, but Santi's sales director didn't care - he simply wanted to see him making more appointments and pressing more flesh, even though it would have resulted in fewer sales.
It can be dangerously seductive. Not selling enough? Call more prospects! But what if someone else has a better product, or your marketing support needs improving? I've seen highly paid salespeople expected to make sales by cold-calling with no marketing collateral, not even a corporate brochure.
'Business is a numbers game,' it's said. No, it's not. It's lots of different numbers games. In some, the odds are in your favour; in others, they're against you. Work rates matter less than the type of games you choose to play. If you calculate the odds, it's easier to beat them.