I'm not entirely sure why such an enchant as Armsman would yeild 0 points. I really don't have any statistics proving it's awesome - its mainly a query. And to be fair I'm working on the assumption that because it was one of the finale enchantments to learn, it should be worth something..
I do find it useful as most of my gear is DPS but when required I offspec to frost-offtank and switch out a couple of tanking-items to increase defense a little, the 2% extra threat is pretty useful for a quick aggro, especially as its not my main build.
Happy to be told its a parp enchant, as long as you do so nicely