If Hillary! were the sort of dedicated Democrat who would ever sacrifice for her party, your point might have some validity. If Dean is running, say, 13% behind Bush, but Hillary! could run just 5% behind Bush, you are correct that that could save a few critical Democrat seats in the Senate -- and preserve the "filibuster" margin, which is all the Democrats have left in Washington.
But, that assumes that Hillary! would ever consider sacrificing herself in an election she couldn't win, for the benefit of the Democratic Party. That woman would NEVER sacrifice herself for anyone else, any time, for any reason, in this universe. That's why I didn't even discuss the Hillary! runs expecting-to-do-good-but-not-win scenario.
John / Billybob
But it might not be a self-sacrifice if 1) she "saved" the Party from extinction (the media would help her make that claim), and 2) she kept Dean from wresting control of the DNC. It would be sold to her as a minimum-downside gamble.