I've been playing the rally game since 1974, and been a licensed ham since 1982, so being one of the resident old farts here on both counts I feel qualified to add my two pence...
Get a copy of the FCC rules and read them VERY VERY carefully, especially the section regarding permissible communications (financial reward etc). Some folk might consider arranging for vehicle service over ham radio a gray area. I've always been mildly concerned about this, and some practices (that are NOT widespread) concerning how scores get relayed. My concerns are known by those in charge in the places in question, and while opinions vary, efforts to err on the side of legality are the rule as best as I can tell.
Generally speaking, though, ham radio is one of the points that helped pro rally break through to where it is now. Ask Jon Davis about the old days, when the field and the workers got sent out into the forest and nobody knew where anyone was until the next morning...
One of the biggest things to remember is how many people out there are listening to you. The scanner community is widespread, and the opinion they hold about us is formed solely by what they hear. Someone acting like the southern end of a northbound horse can ruin it for lots of people for a long long time...
Having that license, having five watts' worth of radio stuck away somewhere padded, AND KNOWING HOW TO USE IT PROPERLY are incalculably valuable out on the stages. Just ask anyone who's been down Missouri way the last couple of years...