Barr may actually swing libertarian on some issues (he has expressed reservations about some the actions of the Ashcroft DOJ in regards to the war on terror), but on the drug issue he is an arch-Drug Warrior. No matter what libertarian stances Barr may have taken on other issues, his fascism in regards to the INSANE WOsD simply cannot be reconciled by any principled libertarian - that's why the LP opposes him.
There are hundreds in Congress who support the insane war on drugs. Why focus on Barr?