Is it significantly better than the rapier or cutlass? It's more accurate, but once you go over 100% that doesn't help. Armour penetration and reaction distruption are useful, but taking nearly twice as long to use is less so.
Using Max Gal Stats (ignoring armor modifications, injuries, etc):
Rapier - 20 base damage (cutting), 100% accuracy modifier, 9 TUs, MELEE*0.4 Power bonus - Estimated Hit % = 140, Est. Average Damage = 76
Cutlass - 30 base dmg (cutting), 90% acc. mod., 10 TUs, STR*0.5+MELEE*0.2 Pwr bonus - Est. Hit % = 126, Est. Avg. Dmg = 98
Spear - 30 base dmg (piercing), 120% acc. mod., 16 TUs, MELEE*0.2+THROW*0.2 Pwr bonus - Est. Hit % = 168, Est. Avg. Dmg = 84
Assuming all equations are correct.
I believe cutting tends to be a more common resistance for enemies, whereas inversely most have either no resistance or may have a vulnerability to piercing. (By no resistance, I mean 100% aka "normal" modifier.)
Spear is two-handed, so if other hand is occupied the hit % will be lower, but due to the high acc % nature, it generally is still very high.
Also keep in mind that rapiers/cutlasses have to overcome armor and if target isn't dead, deal with retaliation (or reactions if Dioxine alters reactions with extended oXCOM option to allow other actions to trigger reactions, notably melee. Currently, once in melee range, melee attacks should not trigger reactions. The extended oXCOM reactions change would allow melee attacks to trigger reactions, allowing for much riskier melee engagements with targets, as well as "swashbuckling" - trading melee attacks sounds cool!).
Spear may actually result in "more" damage (that is, consistently), as it can ignore 20% of a target's armor, not to mention helpful for rookie training with the 120% accuracy modifier, as well as draining target's TUs for reactions (not sure if it affects TUs on enemy turn at all?).
In the end, each is HIGHLY dependent on the Gal's stats. If anything, Spear should not be given to low TU Gals due to the non-scaling attack cost.