A good buyer's agent is worth their weight in gold. They know the area, know the market, and they watch houses come and go. That insight can be very valuable. We just worked with a very good one. Best realtor experience we've had. Crappy ones that are mailing it in are paperweights. I bet they can get jaded by wasting day after day with tire kickers, though.
I think that a person can get away with FSBO in certain markets. We did it with a townhouse way back in the day. Was a real easy win-win for us and the buyer because it was a simple transaction. I do think a lot of FSBO's tend to be people who think the market analysis was crap, and they don't want to pay out 6%. Of course, in an underwater (or close to it) situation, the reluctance to go with a realtor is even greater. When you're coming out ahead, it's easy to share the profits. When you're selling a house that you owe $300,000 on for $300,000, you're not coming out even, you've still got to pay some guy $18,000 out of your pocket. That can be hard for people to swallow, even impossible in some cases. Their only choice is to go FSBO, or raise their price enough to cover the commission.