Ethics or Convenience

On the topic of farm subsidies in this and this post, my roommate asks the question:

So do you think it is wrong for Jimmy Carter to accept farm subsidies?

My answer is no. But I think it brings up an interesting ethical discussion.

I don’t see a problem with opposing a policy but not rejecting its benefits. So, by my ethics, its okay to want to do away with agriculture price supports, but still accept subsidies while that remains the current policy.

To provide another example, I think the home mortgage interest deduction is a bad policy, but I sure as hell am going to deduct the interest on my home loan as long as its in the tax code. And a different example, on occasion progressive democrates are berated for wanting higher marginal tax rates when they are not voluntarily writing a larger check to the IRS. It seems to me that this attack is the equivalent of monkeys throwing feces.

Though perhaps this could also be an ethics of convenience. Is there a stong argument for why it is never justified to oppose a policy while simultaneously accepting in benefits?