The highest producing agents are indifferent negotiators, but work hard to convince you they got you a great deal. They are trying to get you to buy quickly. Every hour they spend showing you more houses dilutes their commission on an hourly basis. Plus if you think you got a great deal you will refer your friends.
Moreover, there is a moral hazard in negotiating. The higher the sale price and the quicker the negotiation is over, the more and more quickly the agent gets paid. The agents I worked with generally did not give off the vibe that they were phased by this. I think at the scale of what a normal person can afford real-estate-wise, the difference is not huge. My latest agent in fact insisted that I go see more properties before making offers when I said I was ready to go. During this time I found the property I finally purchased, and it was about $50k cheaper than the original property I was interested in.
