One objection is that "economic conservatism" and "fiscal conservatism" are different things. One can be socially liberal and fiscally conservative, in the sense that you're only willing to constrain your statist do-goodery to the extent you're able to pay for it. This is certainly an intellectually defensible position.
But politically, this is hard ground to defend. It turns out that people who buy into the logic of social liberalism, not just on abortion but racial and other issues as well, usually find themselves ill-equipped ideologically to say no to additional spending on causes they care about. They even find it difficult to stay Republicans, as we can see from recent example Colin Powell, who endorsed Barack Obama for president for largely ethereal reasons.
It should be noted that it's also difficult to be fiscally conservative and socially conservative if you've jettisoned the conservative dogma of limited government. We saw this in spades as President Bush embraced "activist government" and ended up wildly increasing government spending over the last eight years.
And that should serve as a warning to those, on the right and left, who would like to see the GOP defenestrate millions of actual, living, breathing members of the party -- e.g., social conservatives -- in order to woo millions of largely nonexistent jackalopes. The GOP would simply cease to exist as a viable party without the support of social and religious conservatives. But not so the other way around. We've seen what happens in this country when the passionately religious abandon love for limited government and instead embrace social liberalism and government activism. The results have been good, as in the abolition movement. And the results have been more mixed, like during Prohibition and the Progressive Movement.
The religious right is much more likely to stop being "right" than stop being religious. And secular conservatives and libertarians who passionately believe in limited government should be very grateful indeed that most of today's religious conservatives believe in it, too.