The problem is, Rothenberg seriously embarrassed himself looking like a Democrat shill with that list. His failure to include more than "1" Democrat top 10 vulnerable House members (when I could name 3 just off the top of my head, Bean, Barrow & Marshall of GA - the latter of which I regard as even MORE vulnerable than Bean) was either sloppy at best, or indicates the man has jumped the shark to the Democrat camp.
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.
IMHO, Barrow is certainly not more vulnerable than any of the ten on that list. The only three Democrat incumbents that one could make a serious argument for are Bean, Marshall, and Chet Edwards.