Yup, the xBow is trainable for the Armsman. As for being a waste of points, I am thinking not at all (having played various archers).
1) Crossbow is a trainable style just as Polearm, Slash, Shield, etc. Therefore, spec'ing in xBow will increase maximum _average_ damage.
2) Also mentioned above, Armsman should be giddy with spec points. Taking into account potential item and RvR rewards, an Armsman could spec to (by 50, and not including auto-training, if any)
39 Slash, 42 Shield, 32 Parry, 40 XBow...
Potentially = 50, 50, 43+, 50+
*Again, this is not counting auto-training. Also, another reminder: the added skill pts (from items and/or RvR rank rewards) give bonuses to hit/dam, not Styles.
Think I might be rolling an Armsman as (another) alt, to see how a pure Tank plays. : )