I wouldn't put it as strongly as Kevin Drum does, but I think this post points to some real questions about whether Ron Paul is tarnishing the idea of non-interventionism:
He's not the first or only person opposed to pre-emptive wars, after all, and his occasional denouncements of interventionism are hardly making this a hot topic of conversation among the masses. In fact, to the extent that his foreign policy views aren't simply being ignored, I'd guess that the only thing he's accomplishing is to make non-interventionism even more of a fringe view in American politics than it already is. Crackpots don't make good messengers.
Now, if you literally think that Ron Paul's views on drugs and national security are so important that they outweigh all of this — multiple decades of unmitigated crackpottery, cynical fear-mongering, and attitudes toward social welfare so retrograde they make Rick Perry look progressive — and if you've somehow convinced yourself that non-interventionism has no other significant voices except Ron Paul — well, if that's the case, then maybe you should be happy to count Paul as an ally.
But here's the thing, if you support a non-interventionist foreign policy (or more precisely, a less interventionist one) what do you do? As Andrew Sullivan notes, there is literally no other candidate in either party that represents your views.