why?  those who will vote for Nader are not "taking away" votes from whomever wins the Democratic nomination - no third party candidate does that.  what a third party candidate does do, however, is provide a voting option for those whose views and issues are not supported or hell, even addressed by the Republicans and Democrats.  Nader voters would not vote Democrat even if he wasn't running, they'd either find another third party (Greens?) or abstain from voting.

third party candidates are vital to democracy.  unfortunately, our political system has gotten so completely out of hand that third parties currently have little chance of success even on a local level.  Nader is the strongest person to fight that or at least bring it to national attention.  i find it disgusting how Democrats used extensive litigation to keep Nader (and others, such as Carl Romanelli - who ran for senate) off ballots and also sued for their incurred court costs.

of course, even if Nader received any significant support, he'd be shut down in the same manner that Kucinich and Paul were - in the form of corporate sponsored exclusion from national debates and media coverage.