All that said, I'm not blind to the detracting points against a Paul presidential candidacy in particular from a Progressive-liberal angle, particularly on foreign policy.
Paul's rhetorical and political opposition to multi-lateral diplomacy and liberal international theories does tend to be a bit much neo-isolationism. I have no problem with the UN or WTO or World Bank existence and US participation. The opposition I have is more that these institutions aren't always particularly effective, but they do have uses. One of which would be that it is possible to shunt off the cost of enforcing any global free trade status and other American interests onto other nation-states. That is to say: that they make defence cuts possible by potentially increasing the share of global defence paid by relative allies (or at least, nation-states that share similar interests that can be relied upon to pursue those interests).
Realist critiques of these institutions to me are more effective than the paleo-conservative (ie, Paulite) demand for stronger isolationism. It's fair to say that they're not always well enforced agreements or weak at best and that especially powerful nation-states will and still do pursue their own interests independent of any limitations. It's not fair to say that they shouldn't exist or that we shouldn't bother to participate in them at all. We might as well try to use them to pursue our interests. The cost is minimal and it might save us some other costs down the line (fewer wars of choice, less cost in defence spending, etc).
Some problems with this critique however. Many liberals tend to adhere to a use of liberal internationalism that occasionally strays far too close to the neoconservative behaviors they claim to abhor and for which a dose of Paul's paleo-conservativism would be a lot better to support. We've gotten the interventionism, and generally similarly useless interventions to their hated Iraq or Afghanistan, from them in places like Libya, Somalia, Bosnia, Kosovo, Haiti, Iraq, and in a "better late than never award", Rwanda, and which even continues to support intervention Afghanistan for "humanitarian" reasons. Their distinction is that somehow there's a process of multilateral diplomacy that legitimizes these as opposed to the supposedly un-legitimate use of unilateral force(of course, except in Kosovo and Haiti there was no multilateral legitimizing either).
I have no problem with multilateral or unilateral power uses, but only in pursuit of legitimate state interests. Humanitarian aims shouldn't require using occupational military force to create, or else such things have costs that should be factored in (ie, civilian deaths and property damage caused by military force) and complications in the use of military power for counter-insurgency uses like that. They're not trained for it, and it's really, really, really hard. Liberals should take that into account before blindly supporting such interventions, and take into account that using our military power for these ends, instead of legitimate state business like defending against or deterring wars of aggression, is incompatible with any demands to cut defence spending. The Paul critique might be too high on rhetorical opposition to engagement with the world (protecting the existence of global free trade requires a little more engagement than mere diplomatic offices.. of course, I'm not entirely sure Paul's a major free trade guy either, given his positions on immigration), and that's a serious problem for any liberal-moderate support to be gained by him, but it's a lot longer on asking difficult questions about what the proper use of such force is than his liberal critics have been.