I do not see a reason why msmtp should block other mail-mta/ packages... Some people might want to use custom smtp sender that's easily configurable by user itself.
I concur. msmtp is useful on a per-user basis, and it aught to be possible to install a real MTA alongside of msmtp.
I should have read more carefully before writing my previous comment. Disabling the mta use flag gives exactly the behavior that I want. I.E., msmtp does not block other MTAs. As I see it, this issue is resolved.
Closing because you can disable the mta USE flag if you don't want it to act as a system's mta.