"We're all born under the same sky and on the same earth. Therefore, we all deserve the same amount of respect." TryxusoftheUndyingSong - Warden - PC/EU
No, they thought it would make it easier on brand new players if they hid a bunch of skill lines that had to be unlocked by doing this or that. I don't know, I wasn't on board with the idea myself.
PC-NA - formerly, mommadani907Guild: Weeping Angels - Co-GMTwitter: @ Loralai_907 several Alt accounts....CP 1700+
There would have been a clear (well I guess not so clear) tooltip pop-up when your character was in the tutorial the first time you put armour on (Or wherever your character is when they put on armour for the first time). It tells you when you wear three of any one piece of armour it unlocks the skill line. It can be easy to miss though (I know a lot of players don't bother reading those when they billboard on their screen).
They should. Hidden skill lines can still gain XP. The obvious one is that you'll still gain XP on an armor skill line if you've never equipped enough pieces for it to become visible. That's the only one I know has this behavior.