How do you come to that conclusion? The field of politics has to find answers for many different question: e.g. fiscal conservatism vs. more debt-based spending, gay-marriage vs. no gay-marriage. Why do you consider it insane to agree with one party on one topic and with another party on another topic? Requiring full commitment to one true party line seems to be the insane position to me.
First, supporting the Republican party for fiscal reasons necessarily means that you're supporting their social positions as well. To paraphrase the old line, what do you call someone who only voted for the Nazis because they supported their economic policies? A Nazi.
In other words, the social policies of a party, especially insofar as they affect the repression of minority groups, a permanent and undeniable stain upon any degree of economic progress that they may also create through their policies.
To put it yet another, more personal way: If you were in a group that the Republicans have targeted with hateful rhetoric or legislation in the last 20 years (gay, trans, Muslim, Latino, etc.), would you forgive me for voting for them if I rationalized it by saying "Well, I think their policies are better for the economy. Sorry that you can't get married / are facing discrimination in your daily life. Thoughts and prayers!"
Second, the data does not support the theory that Republicans are better for the economy. Under Republicans, GDP on average does far worse, recessions are more likely, employment goes down, debt and deficit go up... by just about any metric, they are simply not very good at governing the economy.
So if someone says they vote for Republicans for economic reasons, it's because they like the Republican lines on 'lower taxes' and 'personal responsibility' at a dogmatic level, not because they've done any kind of thoughtful analysis.
