> We do that so that you can't game the system with the same email address. + are invalid.
I'd urge you to reconsider.
I can understand why you'd do that, but honestly I don't think it'll help much. Anyone who wants to "game the system" will probably just use a couple throw-away addresses from a disposable address service or something similar.
Further, blocking the use of that character is an inconvenience to people like myself who use it to track where their mail comes from (i.e. if I get spam to 'mybox+launchrock@example.com', I know exactly who leaked/sold my address.)
Plus addressing is not very effective at preventing spam. Any spammer knows to throw away everything after the plus sign so that you can't track it.
I use multiple email addresses on my domain to track this kind of stuff. Also makes it so that you don't have to worry about sites not supporting plus addressing.
