When the players are using it against the bad guys, sure, Bestow Curse isn't that powerful.
Now have the bad guys start flinging it at the players. A spellcaster can be crippled with this spell. for kicks, try targeting the cleric's wisdom with it sometime. If you get through their Will save, not only will they have their spell power greatly reduced, you might hit them hard enough to leave them unable to prepare remove curse.
I could see bringing it down a bit, blindness deafness is a comparable spell with a similar permanent effect (try using magic missile when you're blind), but if so, then remove curse should be lowered accordingly.
Bestow Curse is in the same vein as feeblemind, it's not designed to kill someone by doing damage, it's designed to cripple their capabilities. I hit the party's barbarian with the 50% inaction version one time. Bringing his hit points down would've taken way too long and required a lot of heavy firepower. This spell crippled him to the point where he got to act 2 rounds out of 5. In a pitched battle, where that one character was the big gun, that made a bigger difference than the 17.5 damage a fireball would've done.