I believe that adding one more application permission that grants access to the user’s e-mail, can only simplify the life to developers and companies that wish to integrate their applications with Twitter.
During the sign-in with Twitter, If the end-user grants the access to its email address, then the process concludes successfully, else the third party application simply notifies the end-user that it has to grant access to its email for the process to conclude successfully.
Many applications require an email address to deliver important information to their users. If after signing with Twitter, they’ll have yet to ask the user for a valid e-mail address which will have in turn to be someway confirmed, it becomes a mess! So, for such applications, the sign-in with Twitter, does not provide any advantage if compared to a classic registration.
Honestly, I don’t see any reason why the end-user shouldn’t be able to chose whether to share its email or not 
Cheers,
Francesco Pipita