Great, but with this caveat: Tom Coburn and Jim DeMint are going to win. They should be working for candidates who really need the help, even if they're less pure. Politics is a game of inches, and some inches actually matter.
The only time that I would seriously work for or donate to a candidate who doesn't closely reflect my convictions is if I know or have the opportunity to get to know that candidate (i.e. local politics), and feel that I will have the opportunity to communicate my views and concerns to him after they are elected.
I will agree with you on this, though -- in general, if a candidate is sure to win or sure to lose, one's efforts are better spent on competitive races where the good guy needs help. I would just say that one can find candidates who fit this bill AND who light one's fire a little. Perhaps a candidate for Congress or the State Legislature who is in a neighboring district rather than one's own. Perhaps in a low-profile state office race...
I feel that those of us who are politically active should always be "on the hunt" for great conservative candidates who are going to be able to go to the next level of politics in our states. I'll vote for anyone I agree with -- my more serious money goes to candidates I agree with AND who have the personal characteristics and energy that are necessary to take it to the next level. While I'll take a lukewarm conservative over a Dem anyday, I'm not particularly keen on working to promote either one's political career.