That works OK until you hit a site that has a weird method of processing payments, and now you can't pay for your order from [email protected] because your paypal address is [email protected] and they have to "match" (and ofc you had no way of knowing this before signing up). Or the sites that don't think "+" is a valid email address character. Or the sites that strip out the "+blabla" because they've decided they're too special for filtering.