Giving the search engines visibility into the effectiveness of keyword buys could potentially allow them to consciously inflate their rates – Google and Overture will recommend a company's most successful words to their competitor, setting off small bidding wars.Andrew Goodman seconds the motion over at his Traffick blog
Bottom-line: Don't give your data to the ad network. Someday it may be used against you. Use 3rd party or in-house tracking tools instead.