Depending on your email provider, there are a ton of different ways to create brand new email addresses. Blocking off the single pattern of foo+bar@example.com just punishes legitimate users without causing any problem for spammers. Besides the fact that foo+bar@example.com may in fact be a legitimate unique email address, there's also just the case of people who routinely use username+sitename@example.com when signing up for anything as a way of tracking where incoming email is coming from.