Edit: doesn't mean everyone will use it though! But guess the plus sign in addresses would be more "email" than just email.
I know someone who constantly complained that web site X or company Y are stupid, because they don't follow the RFCs, don't know the syntax of mail addresses, because mail validation in web forms often rejected anything with a plus sign.
The correct answer would have been "don't do it then" or maybe "how about configuring your Exim so that instead of '+' you're using '-' as a separator, but I suppose the complaining was a big part of the fun.
There's also a generational memory issue here, and I'm not aware of any C.S./C.E. programs that cover RFCs as part of the core curriculum.
(viz. not a gmail-specific feature)
the plus syntax is part of the email address specification. any server that doesn't support it is by definition buggy because some mail won't work as expected or designed.
Many mail servers have a setting to configure which character to use; some default (or used to default?) to "-" instead of "+".
