Hey! Crossbows don't add an extra attack & take two hands to wield, yet I don't see many folks calling them underpowered. The item upgrade throwing spears give better equipped bonuses than any bows, have better enchantment bonuses than most arrows, never run out of ammo, & do good damage. All in all with the extra attack they are superior to bows. Without it they would be even.
I call crossbows severely underpowered. In real life, they completely demolish bows. In AD&D, they're often pathetic by comparison (and let's not even bring up the sling of seeking).
Better equipment bonuses? Only the Unicorn one gives equipped abilities, and that one specifically does not give the extra attack. By the way, the Mana Bow equipped ability is vastly non-trivial. Too bad the damage bonus is so low.
A +3 throwing spear from Item Upgrade is not significantly better in terms of enchantment than a +3 Quiver of Plenty from Item Upgrade. Neither run out of ammo.
Without the extra attack they would be strictly inferior to throwing axes. Throwing axes do the same damage (1d6) and would have the same rate of fire (1) but take one hand, so you can use a sheild. That's an incredible advantage. Think Shield of the Order, Harmony or Balduran here. Think about Rifthome, Dwarven Thrower or K'logarath here.
In order to make the spears not strictly inferior to other thrown weapons they do need some sort of help. Remember also that bows give extra flexibility: you can use normal missiles to get through pro-magic-weapons, you can use arrows of dispelling, you can use arrows of detonation (yes, you can get all of this in BG2), you can do elemental damage to kill trolls. Throwing Spears get none of that.