Great sales people are a game changer. People that can sell your vision and convince the audience that the problem you're solving is the biggest issue their business faces. They're completely indispensable. Bad sales guys, the type I've usually come across, bring too little domain and product knowledge to the table, and end up selling the completely wrong thing if they sell at all. Like Ben Horowitz said, great sales people protect your business [1], they don't wreak unnecessary havoc on it.
[1] http://bhorowitz.com/2010/08/29/the-right-kind-of-ambition-2...