As long as people are raising private money in big numbers to finance expensive campaigns, you're going to have big contributors who have special interests controlling the outcome, whether it's video poker or the hog farmers or tobacco companies or the trial lawyers.
These are the kinds of issues that whip up people in primary elections. The turnout is going to be really low, so I think these candidates are looking for something that whips up the faithful and increases the turnout for them.
It gives you access to Republican interest-group money that you don't have if you don't take the pledge.
It puts them in a very precarious position. They know when they take the pledge that it's not in the public's interest.