I've been around here for a while (check my UID). I remember back in the 90's when they explained this - and it never made sense to me then, nor does it make sense to me now.
Reddit proved that editing/deleting posts doesn't have those kinds of issues.
Even if you accept that argument, getting around it by instituting some sort of versioning for posts - ("This post was edited. Click here to see previous versions.") can eliminate that.