I think that a lot of Democrats will be voting for McCain not necessarily to create mischief. What conservatives have labeled as betrayal, liberals and independents see as principled stands on McCain's part. Several of the Democrats will be voting for McCain out of respect. This would be similar to me, if the Republican primary was meaningless, voting for a Lieberman or a Moynihan in the Democrat primaries because I respect the man (not necessarily their views).
In all likelihood, McCain is emerging as the strongest general election candidate which the polling has shown for a while. It would be potentially suicidal for the Democrats to promote the McCain candidancy. His appeal to independents in the general election could hard to overcome.