I respecced at Level 55 to try out music and dance to work out what it did. I put a bunch of points into the Captaineering tree, survival, armour and melee etc just to make sure I hit all the points I really, really need before I start risking my skill points on this tree I didn't quite trust.
As it turned out, I was quite happy with the accordian's buffs, though the war drum is broken (which isn't the topic of this bug report) I noticed that I didn't have access to the captaineering skill line. I thought this was really strange because I remembered speccing into it before music, but then I noticed that I still had the "Rally the Troops!" feat in my hotbar. I then went into my character screen, scrolled down and saw I had all of the Captaineering buffs i'd specced into. It's just I'd lost the "Captaineering Unlock Unlocks: Captaineering" Perk from the Seamanship skill tree, locking me out of the tab but still keeping the perks I had.
Okay, I thought. It's just two skill points. I'll re-purchase the perk, it's probably a one-off event. I repurchased it, all of the perks i'd previously purchased before the skill tree had been locked were still there, so I moved on. I noticed that the 'Artillery' skill tree had completely replaced the right hand side of my skill book, but I didn't know what to make of that, so I just ignored it and moved on. It never went away.
Then I noticed this morning that I was locked out of the Beastmastery, Melee Weaponry and Armory Traits.
http://i68.tinypic.com/sc51lf.jpg
And still, as you can see, I have the nature's call and touch feats still available.
http://i64.tinypic.com/2uhpsua.jpg
And I'm still wearing fine quality armour, and still able to craft swords.
http://i63.tinypic.com/2m2cjs4.jpg
I'm about to respec, and I'll update this thread if it fixes it/makes it worse or if this issue reoccurs.