But if someone is not interested in notifications and is only going to ignore them or trash them, then forcing them (whether it's for the initial comment or the edits) serves only to annoy the person, and not to catch spam (as they're not going to read them). So it doesn't really accomplish the goal in any case.

I actually do like getting comment notifications and edit notifications and wouldn't turn them off, but someone who doesn't will not be reading them either way. (And trust me, the on-site inbox can be completely ignored if you want to. In fact, I believe that is the best way to use it, as I deeply dislike the thing. I send things I want to read to email. And as noted above, email can filter the things and toss edits into spam if you really want. Or everything.)