I think they used to be very important. Candidates weren't covered 24x7 and didn't have as many outlets for the message as they do today.
While I think the news media is vital to our country and that many sources do try to provide an unbiased account, there's now an emphasis on the entertainment value of the news. The news has to be exciting to get people to watch, and the number of people watching relates to revenue. Consequently, the debates have become more about scoring a point off the other guy than a discussion of the issues. So if anything, they're even more like boxing matches than they used to be.
(Remember the Bentsen/Quayle debate? "Senator, you're no Jack Kennedy..." LOL. That was a pretty good one. Guess I'm dating myself!)