I have a two-part question:
What reasons are there to not have a RE agent as a prospective purchaser? Provided there is a property you want to purchase and are comfortable with negotiation, I can see possibly negotiating a lower commission and home sale price by going to the listing agent. The main value I see in an agent is understanding of the logistics, list of people to call for inspections and related items, and understanding of comparisons to know the value of homes on the market (though recent sale history can help with that a lot and is available through site like zillow and most county tax assessor pages).
How would you select a RE agent to represent you? Many people I know selected an agent based on their friend's sister being an agent or some such. This does not seem like a good metric. Neither am I interested in the agent who simply closes the most deals as I'm not sure that means that parties are being well represented (they may be, but I'm not sure that's a good metric).
I have a strong tendency towards DIY, but would like a more experienced opinion of the pros and cons. My take is that the right agent is worthwhile as a purchaser throughout the process.