Nobody expects 100% agreement on every issue, that would be silly.
However, there are issues that are deal breakers even if that issue is the only issue you disagree with. At this point I think it's healthy to be exploring all angles of every issue. 2012 is a long way off.
I certainly agree that it would be good if one to be elected is 100% on the conviction, yet suppose you have a conservative who is not 100% and a liberal who is 100% in the other direction? Do you not vote for the conservative? If you do vote for the conservative you knock out one liberal vote, but if you abstain, you allow the liberal to his or her vote.
Now is you insist that our candidate is 100% this is a different issue, as you can insist on a 100% conviction, you can make it clear that you will not vote for one who is not 100%. I agree with that.
On the other hand, if you have only two choices and neither is ideal, but one far better than the other, then it makes sense to, in essence, to vote against the liberal.
Further, you can abstain and it does have merit; frankly I might do this, but in the past, I saw it more logical to choose the lesser of two evils, or is that weasels?