There are actually not that many messages to be sent: Yesterday there were 246 changes on user talk pages. Some of them were edited several times, some were edited by the users themselves. So it might be about 150 messages - a surprisingly low number, I think.I guess the problem of email notification could be a lot of emails being sent.
But compared to single user login (SUL), email notification is ready to use (according to Enotif on meta). Who knows when SUL is going to be implemented? Until then it is useful to reach users of other Wikipedias that only come to commons only to upload an image every once in a while.But email notification wouldn't be necessary. When the Single User Login gets implemented, it'd be easy to have a second "You have new messages" text.