IRS does not like to expend time and effort on a case which will result in no additional tax. Even if you are familiar with the law and IRS' procedure, it is a good idea to get someone that is personally removed from the issues involved. As the taxpayer whose pocket is being picked, you are not in a good position to distance yourself from the situation. This may result in poor judgment calls on your part, or worst yet, anger at the Agent. If the professional relationship breaks down, the Agent may become polarized and entrenched in factual or legal minutiae that could result in a less conciliatory stance.
Some Agents are more difficult to deal with than others.
The more difficult ones may make unreasonable demands for evidence or may want to probe beyond the narrow issues involved in the audit. Without representation, you may not know how far the Agent's inquiries may go and disclose information that may lead to a broader audit or even a referral of your case to the Criminal Investigation Division. Some issues, such as penalties, may be negotiated. Sometimes the Agents assume a quid pro quo stance and propose to concede some penalties in exchange for concessions on the part of the taxpayer. Without a competent representative you may not be able to judge how strong the IRS' position is or which offers are worthy of consideration.