Because there are a lot of people on this list who are interested in spreading information on the wikis, which overlaps with an interest in MassMessage:
In
https://phabricator.wikimedia.org/T270435 there's a discussion about how to be able to use MassMessage when using the new "Page to be sent as a message" function, which would require an easier way of adding a timestamp. My suggestion is to add it automatically, since MassMassages are going out to discussion pages of different kinds, and should be timestamped. If you have a workflow where you don't want your message to be timestamped, please let me know off-list or in the Phabricator discussion.