In marketing we can view the sales team as our customers really, and we all know how crucial it is to understand customers. The problem at many companies (especially where the sales team significantly outnumbers the marketing team) is often that the sales team shout loudly about what they want, but the marketing team "know" that the sales team doesn't really understand the problem. Immediately you get a "them and us" situation, and I can name many companies where I've seen this happen. So often marketing is a gentler, more thoughtful environment, while sales is more testosterone-fuelled, and the clash is inevitable.
What both sides have to do is to swallow their pride and work together. Easier said than done? Well, in my marketing role at various online and print publications over the years, I've made a point of going out with the sales guys and seeing what they have to cope with. It was hard to shut up and just observe, but I did. In the first magazine I worked on, I shared an office with the sales team and I could hear them being rebuffed on the phone all day; this quickly gave me a good understanding of the objections they were coming up against, and they weren't what I might have guessed.
This is covered in Eating Nails for Breakfast: 2 Weeks with Sales and How It Made Me a Better Marketer on The Funnelholic where the author gives you five ways to observe sales at close hand. Maybe it's an idea for the new year.