I've heard a few interesting arguments (mostly by Gilad Bracha) against type-dependent semantics, though I don't think these arguments usually apply in this sit

I've heard a few interesting arguments (mostly by Gilad Bracha) against type-dependent semantics, though I don't think these arguments usually apply in this situation where you're just talking about polymorphism in (+). The argument is that if your language doesn't have type-dependent semantics then you can plug and play type systems as completely independent static analysis tools. I don't know that I buy that those tradeoffs are best, but there is something really interesting there.