Great deals aren't usually listed in the MLS. MLS is for marketing the properties that I find to the general public (and sometimes I will use a realtor for that).
Why would I hire someone to help me structure a deal when that's supposed to be my expertise? Not trying to be rude to all realtors because I am sure that some of them are very skilled in structuring deals. But as an investor I've got a pretty good idea of the options available to me.
I like to see what my competition is doing. I recently spoke with a realtor that was selling a rehabbed house for a private investor. I started talking with the realtor about signing a lease-option contract for the house. "I've been a realtor for 20 years and I've never done a deal like that. Maybe you can talk to the owner?" She then proceeded to call the owner and I spoke with him for a little bit. His response? "I'm actually pretty new and I don't know anything about lease-options. Why don't you talk to my realtor?"
In any case I would only use a buyer's agent in two situations.
1. If I was new to the type of deal I was doing (a raw land deal, for example)
2. If I was trying to buy property in another state (not going to happen anytime soon, I have enough opportunities around here).