The problem is that [email protected] and [email protected] can be different people, or [email protected] and [email protected]. On gmail, they can't, but on my university email system for example, this is the default naming convension for circa 50,000 people. If you consider them to be the same, one of them can't sign up to your service. How often this happens? Probably not all that often. But if you run a popular service, it will bite a few people.