One consequence of removing the MTA is that mail generated by the system itself (usually sent to root), for example by Cron, Logwatch, rkhunter and others cannot be be delivered.
So yes, an MTA is an integral part of any Unix-like system and I doubt Debian will let you uninstall the MTA without complaining and suggesting an alternative. But it’s not a big deal. By default in Debian even sendmail will only listen on 127.0.0.1 and relay mail only locally, so there is no real security risk here.