Figured I would build a bit on the Jack of all Trades, not to mention related ideas, so to expand on it:
- UristMcSpecialist changes from a Miner to Stonecrafter profession. Later on he starts dabbling in Stone Detailing.
Specialist - Dwarves that focus on a specific set of skills will level in those skills faster. As an example, they may specialize in working in Stone Related jobs such as Mining, Masonry, Stone Detailing, and Stonecrafting and find themselves becoming a Stone Specialist. As a bonus, they increase in rank for those skills faster, while perhaps as a negative, level in outside skills much slower.
It should be noted one might look at jobs in a matrix type of thinking as well. For example, the Stone Specialist Above has several skills that might fill other specialist roles: Plant Gathering, Wood Cutting, Mining, and Fishing could be another set for a Gathering Specialist; Stone Crafting, Wood Crafting, Metal Crafting, Bone Carving, and Wax Working could be another set for a Crafting Specialist; Masonry, Carpentry, Building Designer, and Metalsmith (Whatever Metal Profession Lays Walls) could be yet another for a Building Specialist.
Depending on how it's done, a Dwarf might only be able to become a specialist in one set of skills, or certain sets might not work with each other. This might be important for Military skills since you wouldn't want a Military Dwarf to potentially level slow in Military skills if he's a specialist in something else (unless you have something like an Axe Specialist or a Pick Specialist like set where Military skills are shared with other labors). Perhaps as well, even though you might train a Dwarf in a specific set of skills, if you start training him in another set he may switch over - or the Dwarf simply turns into a Jack of all Trades. As for the Jack of all Trades, that could simply be a catch up for any combination of unrelated skills not in a set.
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Building on the Jack of all Trades - let's say you make your Dwarf carefully so that skills don't rust. If you manage to get him up to the Master skill rank, you might be able to upgrade him from 'Master of None' to 'Master of All'. At that point your Dwarf can beginning leveling up skills quickly to the Master rank before problems begin to occur.
Additionally, if your the type of person who wants to get Dwarves with a bunch of Legendary skills, you can eventually do it since if you combine with the Legendary status I mentioned before, you wouldn't have to worry about the skills degrading at all.