If you want to use a discount buyers agent then you should. The specific reasons you are looking for to hire a full-service agent over a discount agent exist in the actual interviews. I have seen "full -service" agents that don't provide full service. I have seen discount agents that do provide full service. It really comes down to the agent.
If you are not and experienced or knowledgeable buyer a professional agent will do more than just fina a home. Much of what we do is behind the scenes. As Brian mentioned we provide a net against many of the pitfalls that can occur during a transaction. Many times we bring the voice of reason when a transaction is going bad and we save it by working with the other agent/party. Other times it is about knowing the right resource and using our relationships to work out problems. Sometimes we simply know the neighborhood like nobody else does and can spot a value that doesn't exist on the internet.
I don't think the decision is between discount and full service. I think it is about what you need/want an agent to do for you. If your only desire is to save money then hire the cheapest. If you want specific services from your agent, make sure he/she are able to provide them. Call references, ask about what they will do for you as a buyer's agent, ask about how many buyer's they have worked for, ask how many times they had transactions of their last 10 that fell through, ask why they fell through, ask what did their last 10 buyers pay for their house compared to list. Ask about the courses they have taken in the last year to keep up with the market and legal issues. What extra training have they completed.
I don't have a specific reason why you should not hire a discount agent. I do believe most do not know how bad a deal can go until their in the middle of it. Find the discounted professional that will go above and beyond. Not sure that is the answer you are looking for but hope it helps in choosing the right agent, discounted or otherwise.
You'll be making the calls & appointments, arguing with listing agents that "you ARE represented, and don't have to sign a 'no representation' form". But you will save money. Only you can determine if the savings is worth it. Some people are discount brokerage people, who like to "do-it-yourself", and some people are full-service people... there's something out there for everyone.
God, I love this country!
Not sure that any of us can give you EXACT examples of what we as, premium agents would do, because every transaction has its own issues. I can say that as premium agent I would be ready for whatever happens.