Obama does have a legitimate excuse for not doing well, but for some reason they don't want to admit that he's president and doesn't have two weeks to spend on debate prep like Romney did.
Incumbents have a history of getting their clocks cleaned in the first debate (eg. Carter 1980, Reagan 1984, W 2004), but I attribute that to complacency. Notice that Clinton had no problem handling Dole in 1996. They also have a lot of advantages (eg. Air Force One, the world's biggest megaphone, etc.) that the challenger doesn't have. If Obama thought his preparation time would be limited due to the demands of the Presidency, he should have started earlier rather than wait til the last minute to cram like a college student pulling all-nighters.