I have been trying to find good excuses for progressives not to register to vote in the GOP primary for Ron Paul. I have found only two:
1. If there is a competitive race down ballot in which a progressive Democrat stands a chance of winning against a blue dog.
2. Ron Paul is more likely to defeat President Obama in the general election.
On the latter, if you register GOP for the 2012 primary, you can vote for extremist and non-incumbent candidates down ballot so as to increase the chances of more Democratic victories in the general election.