Why should this be included in the repository? Mailpile offers a web interface with full GPG support for a personal email, it also allows one to have multiple accounts and offers an easier way to adopt and use GPG from the start. It is a light weight option for email in an interface that more and more are using, i.e. web.
Is it Open Source: Yes
Who and how many users do you anticipate will use this software? This is hard to speculate, it is in constant development and as many have been seeking an alternative, so I cannot speculate.
Link to source tarball/zip file: https://github.com/mailpile/Mailpile/archive/1.0.0rc2.tar.gz