I constantly struggle with this question Tim.
I really find it hard to value the services of a real estate agent (to the extent they charge), even one I trust and has proven results for us.
We tried to sell two side by side properties ourselves not that long ago as we were struggling with the same issue, we did a nice advert on RE.com, worded well, had quite a few people through, but couldn't get the sale.
Ended up using my Realo to do it for us, and they sold both within a few grand of our price expectations in about 3 weeks.
We were happy to get the sale, but handing over 32k across two sales really irked the hell out of me.
I do believe there is significant value in their brand (both name and company such as Ray White), value in their database (if they have one and work it) and value in their time. Also, some buyers are suspicious of private sales, from my (limited) dealings I believe it is becoming less "feared" but it is still a factor.
Taking all that into account, to me it still doesn't stack up to anywhere near what the vendor pays, especially when normally you pay for marketing (which the agent normally benefits from anyways, with their name and face on RE.com and posters etc).
Because we are always working with our agent, the way we justify it is slightly different to the normal vendor. We view it as a two way street, in that our agent is always working to bring us off market deals with solid returns, and for that we help him earn commission on our purchase, and sale of some of our stock. In the last 18 months, we have bought a number of sites through him, and sold a few individual properties.
For the one off transaction "If you don't ask, you don't get"