I recently watched the documentary "Journeys With George," which follows the experience of a journalist assigned to cover George W. Bush's presidential campaign. What I found the most interesting was the seemingly close relationship that the press corps developed with the candidate, as they traveled together and interacted on a daily basis. It was interesting to see how Bush's interactions with the filmmaker, Alexandra Pelosi, were so mixed, as the film showed them talking about both her crush on a fellow reporter, as well as Bush's attempt to win her vote.
A similar situation occurs in the White House. I think there is a danger in having an assigned press corps to cover the president, because it means information from the White House is being filtered through the same reporters all the time. On top of that, these reporters often spend so much time with the president and his staffers that the lines between personal and professional relationships could potentially be blurred. I can easily see how a reporter could lose his/her perspective on where his/her loyalty is supposed to lie.