Shockingly, I get the idea of variance and how important 5's are. What I'm opposed to is "It is rare that you ever actually want a second Sea Hag, since they are completely useless once the Curses are gone." The fact that Hags are mostly worthless (barring the usual suspects: TfB's, heterophiles, etc.) after Curses are gone is not why we question second hag. As Dondon notes, a second hag will normally give you a 6:4 split and 7:6 dead card split - this is a win. What matters, is the opportunity cost of not buying the second (third, etc.) silver, or the competing 4's or 5's. A second Hag comes at an opportunity cost of a silver (roughly, 1.5x what an Igg runs). Given a choice, in general, would you trash a silver to give a curse?
Okay, so let's posit that we have a mirror with both players opening Silver/Hag. After T4 we most likely have 13 cards in the discard with two being bought and one being flipped by each hag (I'm ignoring the cases where people flip hags or where the hags end up in T5 hands as these are edge cases). At T5, you will have bough two cards and played a Hag. 2 Cards are left in deck. Assuming that if you hit 5, you buy that. This means we have end up with decks decks that must contain 7 Cu, 3 E, 1 Crs, and either 1 H / 3 S or 2 H / 2 S (again ignoring edge cases where you hit <3 coin; I'm also assuming that hitting a power 5 here, means you automatically buy the 5 - in other words we ignore the cases where buying the second Hag doesn't effect outcome). This moves your average card value from .87 coin to .73 or about a 16% reduction. That isn't a lot, and it, going with HME's estimated tempo figures, is as bad as it ever gets.
So we are, really, talking about the opportunity cost of foregoing a single silver, or some other better <=4 coin kingdom card. If there are no other Kingdom cards you really want (e.g. the Kingdom is mostly Woodcutters, Talisman, etc.), sure go for the second Hag. Sure, obviously some fives, like Hunting Party, are pretty much assured single Hags. But some Fives, like Outpost, pretty rarely are worth losing the dead card split 7:6. Where is the tipping point? Something mediocre like Lab? Rabble without village support?
The second Hag being a dead card doesn't matter - if you could get it for free with a Silver (say you are granted a magical single chance to get a free Haggle) you'd almost always want the second Hag; what matters is not buying the Silver (or other better card). I'd like to better delineate where the lines are - some fives make it better to play for them, some fives make it better to run Hag/Money.