Whether it is a good or bad idea, America is not ready for legalized heroin. It is hardly one our pressing issues at the moment and popular support for such a thing simply does not exist. Look, the potheads in California couldn't even get weed legalized - and that is in loopy CA.
There are lots of intellectual (and very interesting) arguments about whether the drug war is a good or bad idea, but this exposes Paul yet again to his fundamental problem - he is a libertarian. The Republican party is a conservative party, not a libertarian party.
So Paul is simply running for President in the wrong party. Now, that wouldn't necessarily be so bad if his running as a Republican meant that when he lost the nomination we could count on most of his supporters to vote for the Republican nominee, but we both know they wont. A large chunk of, perhaps most, paulbots probably go on to vote for libertarian or downright liberal candidates and the Democrat for President. He and his supporters are really just using the Republican party to push his message and to keep his endless presidential campaign going. There is very little value for Republicans, a traditional conservative group of voters, in having Paul in the party at all.