I don't think you're right. I think that if, say, a Witch gave Curses that were +1 VP instead of -1 VP, it'd be a pretty bad card.
You're right that the VP swing given by Curses isn't usually the most compelling part of the attack -- the deck dilution is -- but the deck dilution part is informed by the negative VP part. The $5 Cursers, at least, don't dilute your deck all that fast. What really slows down Cursing games is the obligation of all players to invest in Cursers that are less good at building their own decks than equivalent non-attacks would be. And you have to invest in Cursers because you're worried about getting a big negative VP drain on your deck, and the best defense to that is giving your opponent an equivalent pile of Curses.
Maybe Geronimoo can quickly confirm for us, but I think that if Curses gave +1 VP instead of -1 VP, single-Witch or single-Sea-Hag would lose to single-Smithy, and they might lose to straight up BMU.