I think it also depends on what stage a business is at. I think if you are in an initial high growth period the emphasis should certainly be on the 'buy right now' customers, these are new customers trying out a new company or customers you are taking off a competitor. I think commission structures are more appropriate during this phase, the business still needs to find what type of customer it works best with, and it needs to get a critical mass of these, and this is mostly a numbers game.

However, there is a point where you've reached a certain critical mass, where your happy customers market for you, where your own marketing and presence sells the company. Like Fog Creek, which is now an established and known company. At this point you will do better at growing your customer base through marketing instead of commission driven sales, where customer service to the existing customers will do the selling for you, and changing the pay structure of your sales people will align them better to this approach. It also allows you to convert your customer base slowly to those you know will remain with you for longer. I think you still need to keep statistics on sales performance, and deal with bad performers and promote good performers, but these should be closer to a bonus structure and work over longer timescales, like the rest of the staff.