From my own experience, with the experience I have had with the MLS systems used on the Costa del Sol, they really only mainly benefit the agents and not the buyers.
By applying such high commissions they have effectively helped to kill the market here. And yes, it was quite common for some properties to have 9% commission added.
So if someone bought a property for 200K, then paid 9% commission to the agent that's a final price of 218K. Now, add around 13% on top of that for taxes, notary costs, mortgage fees and costs, etc, and in essence that 200K property is really costing 246K.
If that person then wanted to sell that property on then they would have to start at 246K just to break even. Add on commission, costs, etc and here we go again.
So, my main issue has always been with the levels of commission applied. It was abused for too long, too many people were taken for a ride and now everyone is paying the price.
It's a snowball effect and it's still rolling.
In the US my understanding is that commission levels are much lower so in that respect I think an MLS system can work for the benefit of the buyer, but the bad practices I have seen here have given MLS systems a very bad name. | 
