Good sales people will always be able to sell but, with good marketing support, they will be able to sell more. Think of bricklayers trying to build a wall. If they are standing in quicksand, they won’t be able to build a very high wall as they will sink when they try to reach up to add the next course of bricks. Now put those same bricklayers on a firm foundation and they will be able to build a much higher wall.
Good marketing provides the firm foundation to allow your sales force to excel. The marketing gang will know the strengths and weaknesses of your product or service and they will know the strengths and weaknesses of the competition. A good marketer will also know how the competition are selling and develop effective counter strategies.
Imagine how much more confident and effective a sales person will be if, before making a call, s/he knows the questions that will be asked (and the answers!), the objections that will be raised (and the counters!) and what the competition have said. Further, your sales force as a group will be more effective if the messages, crafted by marketing, are played consistently across all media.
Sales, with Marketing support, is far more effective.