NPC Quest / Skill Training

In several towns I noticed NPCs offer training master / GM of a skill. Example Critical Strike can be learned at the Forge in EL. But you have to complete a quest to get the training and it requires you have Expert in that skill already.

Is there advantage to doing this quest / training? Do I still spend Build Points to advance the skill like if I read the Master book? Why do this instead of just buying the Master / GM skill book?

There was a time, very long ago, where you couldn't just buy these books at every skill shop.

Now that you can there is no real "advantage" to doing the quest other than many of them have unique house item type items in the quest lines (if you don't actually hand the items in to complete the quests)