They should be informative so that a voter can understand how a candidate is inclined to use his leadership skills in policies that will be implemented in office. Therefore, the purpose of an ad is to explain policy issues rather than to portray the character of the person. We are talking about 30 seconds - they want us to vote. If they really wanted to disseminate accurate information they would send us all copies of boring but useful policy papers for us to decide with - not sound bites. No matter what the style of ad, the purpose is to differentiate one candidate from the other and thus make the opponent look less favorable.
Most commercials are lies.
There should be someone who checks the facts and has the power to do something causing a public retraction. Americans are much more concerned with a candidates' character than with his stance on issues.