I can appreciate why an agent would want a buyer to sign an agreement in an effort to keep from wasting their time, and I like the idea of the agent representing my interests in a transaction. However, I've encountered super crappy agents and would hate to be in a position where I'm forced to work with them on a deal they didn't have any effort in.
Just before I bought my residence 6 years ago, I was putting in tons of hours at work which included some travel and didn't have time to surf the internet trying to find a place that met my specialized criteria. I did eventually run across a place I wanted to see and called the listing agent for a showing. On the phone prior to and as we walked through the property I explained what I was looking for in both a property and an agent to help me in my search. She was all peaches and cream and led me to believe that she was "my guy". As we were parting ways, she says, "Just let me know if you find any other places you want to see." I nearly lost it.
It was clear to me then that this particular agent wasn't going to put effort into helping me find a property and that her interest was really just to write up an offer and cash her commission check. Signing a buyer's agreement with here would have made me very unhappy.
There are plenty of good agents out there. Just be sure you vet before you sign.