I love Bill Clinton, but this is a fair point for the Obama campaign to make. The presidency of Bill Clinton was clearly good for the country, but it was horrible for the Democratic party. Furthermore, without Bill Clinton at the center of it all, the party essentially collapsed after his presidency. It wasn't until 2006 and the implementation of Howard Dean's 50-state strategy that the party recovered from its 1990s losses.
I think one of the major concerns for primary voters is a candidate who, once elected, will also have coattails. I think the concerns about Sen. Clinton not having coattails on election day are not valid, but I think it's after that that matters. Many would like to see 60 Democratic senators in order to get this important legislation to pass, and the contest is who can deliver.