it does when the service relies on the human element to get completed.
You pay up front for websites/apps, because you get access/product right away.
But any interaction where the human is involved(plumbers, tree service, mechanics, body shops etc)...you pay only after the job is finished.
The reason is that when you pay someone they have no real incentive to give you their best service...since they already got paid. And yes word of mouth might work, but I'm not seeing awesome deals on there. The venture beat article shows they got someone an Accord for $32K. Carsdirect has that same top of the line trim for $28K.
And carsdirect has the same incentive, give good service to get word of mouth so they can get more leads....the only difference is that you get to save a bunch of money too.