Moreover some broken email validators reject plus addresses. It's nice to be able to fall back on special dot combos when you want to keep track of sites selling your email to spammers.
Have you ever tried it on Yahoo? They wouldn't let me set my email address to yahoo@mydomain a few years ago, claiming that the "email owner" had blocked this: https://www.dropbox.com/s/t213wkajdz5753k/yahoolies.png?dl=0