The thought is that the IRS may be forced into hastily asserting liability to preserve possible claims. Once the IRS is in that posture, it is more difficult to deal with and it might want some blood for the extra adminstrative trouble instead of some more cooperative resolution of any issue of interest. I have only refused to extend the statute once. That was with the Department of Labor when it was so far out of line with its original assertions and general rudeness in the process that things could not get worse. That is not typical of my experience with the DOL. We tend to get constructive behavior from IRS agents, too, so we generally accommodate.