My pick of the useless skills:
(A) Might Tree. For warriors you will end up maxing them anyway.
- Training (lv 1 required) There are much better units than archers very soon. Knights are good against dragons but you don't need the circle attack to hit them effectively.
- Combat Readiness (lv 1 required). Turn One is the least likely turn that you will be hit by anything. For dragons you can use Trap / Glot's armor.
- Frenzy (lv 1 required). When you can wipe out 1 stack you can wipe out 10 stacks.
- Tactics (lv 2). It is always better to wait on turn 1 so you can wipe out the faster units first.
- Spirits of Rage (lv 1 enough). The best rage spirits spells are all available quite early. You don't need to push from level 28 to 30.
(B) Mind Tree. For paladins you will end up maxing most of them anyway.
- Scouting (Lv 1 required). I don't know why people keep recommending them. This is a single player game and you are supposed to reload like crazy.
- Anything that gives extra money (Trade, Trophy, Keeper of the light). Even on Impossible you will run a large profit, if you know how to conserve troops properly and know the right troop to buy.
- Diplomacy (Lv 1 is enough). Your troop count should be always maxed, or close to max since you just pulled a flag.
- Inquisition (Paladin only) - Useful if you don't have sacrifice. Useless if you have it. In my game I didn't. Great skill.
(C) Magic Tree - for mages you will end up maxing many of them anyway
There is no useless skill here!
- Alchemy (Mage only). You can leave this as the lowest priority since 80% of spells are simply not worth learning. Plus the most expensive upgrades only come in end game - so there is no point investing in this early-mid game.
- Necromancy - useful if you like undead and combine it with the might skill Dark Commander. There are so many good living units, though.