Here are a few reasons I think this is a mistake:
1. McCain is encouraging investigation: McCain's recent lobbyist -- and pastor purges -- create an environment that rewards his opponents for continuing to demand McCain oust his friends. There are certainly dozens of McCain operatives, advisers, or staffers who currently work on his campaign, who also are involved with 527s or lobbying, at some level. McCain has shown his enemies that their inquiries will produce results, and what gets rewarded gets accomplished. McCain has also essentially conceded that he is responsible for the comments made by anyone who has endorsed him in the past. If I were an Obama supporter or operative, I would be pouring through the comments and writings of everyone who has endorsed McCain. This, of course, will lead McCain to purge more conservatives -- which will continue to drive a wedge between McCain and his conservative base (assuming he has any conservative base left).
2. Real votes are at stake: Rev. Rod Parsley is a prime example of how McCain's purges could have real electoral consequences. Everyone knows Ohio is the swing state, and it just so happens that Parsley's church is a veritable get-out-the-vote organization. Couple McCain's Parsley purge with his recently condemning a popular Ohio radio host who mentioned Obama's middle name -- and all of a sudden you have a real base problem in Ohio. And oh yeah, Rob Portman won't help you there, either (and if you think he will be enough to appease cultural conservatives, that shows you're out-of-touch with conservatives ...)
3. It strikes me that all McCain's concessions are to the Left (this includes sacking the consultant who headed up his conservative outreach for the primary). Fair enough. If you want to oust anyone who has said or done controversial things, conservatives should expect reciprocity. What about Juan Hernandez? Don't you think it's time to "denounce" him? Note: I think a lot of conservatives forgave McCain for making the infamous "agents of intolerance" remark back in 2000, because it was assumed that McCain was justifiably angry about the push-polling that had been conducted against him in South Carolina. But now it's starting to look like that was the real McCain.
... My belief is that politicians tend to follow the path of least resistance. They operate based on rewards and punishment. So long as appeasing the Left gets McCain ahead, he will do just that. For this reason, it is vital that conservatives hold him accountable and keep him balanced. Conservatives should expect a good Veep pick (not Lindsay Graham, not Crist, etc.) -- as well as for McCain to get rid of controversial influences on the Left, such as the aforementioned Juan Hernandez. Failure to adhere to these minimum requests may have real consequences.