Every time I've tried to use that feature, the email field in the registration form I'm trying to fill out rejects it because they don't like + in an email address. There are a lot of not-quite-correct email form validation routines out there. Or maybe this is selection bias: the forms where I'm most likely to want to use the + are with the companies that are most likely to want to resell my email address, and they may be intentionally rejecting the +.

Lots of programmers try to write regular expressions to validate e-mail addresses, but it's extremely difficult for them to get it right, because valid e-mail addresses as defined by RFCs 822 and 5322 fall outside the set of formal languages describable by most regular expression libraries. See this fun stackoverflow answer [0].

[0] http://stackoverflow.com/a/201378