Just to share some thoughts on this topic. For a long time there has been this debate over whether or not a hammerdin is an effective counter to a BvC. Let me preface this by saying that I play a BvC, and I have very little experience actually using a hammerdin in duels. There is a progression of strategy as I see it, as each class has certain counters to the other one's tricks:
1) A hammerdin that sits still and hammers, or one that teleports around and hammers, will generally kill a barb that teleports on top and does WW. This is most commonly seen in pubs.
2) A good barb that can short WW, make use of the blind spot, and predict desync will generally kill a hammerdin that just hangs around and spams.
3) A hammerdin that makes use of desync, charge and is reasonably defensive will kill a barb that tries to tele-ww him to death, no matter how good the barb is at tele-ww.
4) A barb using Widowmaker can force a hammerdin to go offensive, and he can usually kill him once this happens.
5) A good hammerdin can force damage exchange with an Enigma using barb (even if the barb uses Widowmaker), an exchange that the Enigma barb will usually lose.
6) A barb can use Fortitude and stacked damage setup combined with Widow to force the paladin into a damage exchange he CANNOT win. Good WWs and prediction of desync combined with high AR and damage give the Barb an advantage over the paladin.
Now, this is where the the progession usually ends. We have the mcm vids to demonstrate that #6 does in fact work very well. But, lets take a step back and actually watch the vids.
These paladins all charge a lot. None of them recast Oak, or at best they do it rarely. They all tele on top without stack and hammer. In all these vids, there are only 3 attempts to box the barbarian in with hammerfields. And, these paladins generally drop in 3 or less WWs ("Gc" goes down in two several times). I'm not claiming to be better at BvC than mcm, but I would say that his strategy is quite simple and easily executed. He spams widow until the hammerdin comes in, then short WWs him, or he predicts his (very predictable) desync and long WWs to catch him in charge. There isn't a lot of room for improvement. All this leads to to next step in the progression:
7) A hammerdin that actively maintains his minion stack, teleports much more than he charges, makes use of unsummon lock, and builds up a hammerfield around the low mobility barb will kill a Fort barb much more often than not.
This is mostly based on some recent duels I've had, with (*tekniqz.) and with a few other paladin players, who explained that they had developed this strategy after dying a lot using conventional tactics against Fort barbs. It's partly influenced by the dueling enviroment on West NL, where most GM duels are held in NM. Oak is physical immune in NM, which means it tanks the first and second WW hits reliably. The tele on top trick is mostly to take the widow out of it, the real danger comes from being boxed in with hammers and then telestomped. So, we get to:
8) A barb must use Enigma against a paladin who knows what he's doing. This brings us back to #5, wherein an Enigma BvC must get a lot of hits on the paladin to kill him. The barb still has a chance to win if he WWs correctly and has to play EVEN MORE defensively, but at this point the paladin has the advantage.
IMO this is truth of the matchup. It's not an original idea, SicHalo among others has been saying it for a while. It varies depending on realm (a realm where duels are in Hell is easier on the barb, one with NM duels is an advantage to the hammerdin). And of course, a barb always has a fighting chance.