Rothenberg has been writing about his beliefs about partisan waves and about how when there is a big change in seats, they all fall in one direction. His list fits that point of view.
You're right about Marshall, but know that your call on Barrow is controversial at best; the district is still quite blue for a southern district, a region where Bush ran ahead of the generic Republican ticket for Congress, because Marshall (however he votes) does not come across as John Kerry.
You have your speculation as to his motives, and I have mine. I think Rothenberg has built his career to date on a certain level of integrity and skepticism of party hype, so I think he believes what he wrote.
There was a poll released recently, most likely partisan, showing Collins far behind Marshall. I don't take that kind of poll at face value but I can see how it could infludence his call in the absence of any other factors beyond looking at Bush-Kerry numbers and the racial breakdown of the district.
Yes, my second paragraph switches from Barrow to Marshall in mid-stream.
I wonder if Rothenberg took into account the fact that both the Barrow and Marshall districts were redrawn to undo the Democrat gerrymander, or the likelihood that there will be a landslide for the GOP ticket, which bolsters both Burns & Collins. Under the current lines, obviously both incumbents would be favored.