When having Master Spear, your skill is added to your Attack Bonus, Armor Class and Melee Damage. Also they have good damage rating per se (ordinary spears d9+(0/1/5/9/13), tridents 2d6+(0/6/12), halberds 3d6+(0/4/10) plus d9/d6/d6 when wielding it in both hands), so at higher levels you end dealing fantastical amounts of damage, easily exceeding 100 at level 90. (With some spells of course, but not counting relics.)
Disadvantage: They're slower (better base than swords - 80 - but they don't get faster with better skill). Despite this I think that when wielding a spear you can do better only when wielding a spear and a sword. (According to Peter2, the best combination for dealing melee damage is Poseidon (a relic trident) + Hades (a relic sword).)As per your off-hand considerations: dagger does not get any speed bonus, because the whole combination has the greater recovery of the both weapons used. Now for a little computation: the best ordinary dagger deals 2d3+7 and the best ordinary longsword 3d3+12. Both skills increase only your AB on master, but dagger gives 1/10 (independent of the skill level, as Grayface says) chance of tripling the damage. So when wielding a dagger in your left hand you don't get better speed and in average the dagger causes 1.2*(2d3+7) = 2.4d3 + 8.4. Still not better than the sword with its 3d3+12 and the chance to hit is just the same. So, on the same level of quality, the swords seem to be always better than the daggers (if you can use them of course).
