Having a purely physical damage dagger on offhand does not change your primary weapon damage unless you’re corsair C2 with the Double Weapon Assault buff (with that you get to attack with both weapons at once).
The advantage of having a (pure physical dmg) dagger off-hand is the fact daggers deal pierce damage when you use C attack. So it gives you versatility if you use sword / mace as primary. No real advantage for when you use spear as primary. Yes you can flip your mace/sword to spear to deal pierce damage instead of wearing a dagger, but not every class can use spears.
Using a 2h spear gives you better damage, yes. But you need either Cataphract or Dragoon to equip one, that’s the drawback.
Daggers are quite useful for STR/DEX cleric classes (paladin or monk) that lack the option of having a pierce weapon. Those classes can usually self-heal easily so the lack of the defense from a shield is not a big issue PvM wise.