The Concealment skill comes from professions that often operate in the shadows, like Nightingale, while Tree Tenacity comes from the Druid. This is an active self-protection skill and one of the most practical non-core skills (*) for a Druid. It can convert mana to transform the skin into a bark-like state for a period, increasing natural protection by 50% per level.
This skill is extremely useful when paired with a large number of natural protection amulets and is one of Brand's most commonly used tactical structures in the later stages.
Of course, the main reason is that these two skills are transparently priced at the Royal Geographical Society (*) in Brags. Naturally, young people wouldn't pass up such an opportunity to learn cross-profession skills, especially when tasks are not required.
