compare monument in strength to duchess. Or compare followers to militia (about half the difference is the value of 1vp) 1 vp is a quite significant difference. Great hall also offers a relatively pure example.
Though the main bite of cursing, especially in the early game, cursing gains quite a lot of power from the -1vp. A 7/3 split in cursers, requires 4 extra vp to overcome. That's more than a duchy and thus has the power to decide quite a few endgame situations. Late game curses, in particular, continue to have bite by reducing your score. Otherwise they would be relatively ignorable. Witch/mountebank are powerful enough (in that a terminal draw/terminal silver isn't particularly actively bad for your deck) that they would continue to be very good cards. I'm less convinced that hag/IGG/YW/familiar would be worthwhile (in decreasing order). I agree though, that I can't imagine any card being sufficiently weakened to warrant a price change. It's just important to remember that 1vp is, in fact, highly non-negligible, especially in bulk. (monument is much, much, stronger than duchess)