IMHO, this would be bad. The Cron Daemon would have to sanitize the
input of the crontab each time it checks the file for running
(presumably every minute, unless their is a way of notifying the cron
daemon of a new crontab.)
The default crontab in debian creates a file in /tmp, the user modifies
it using their favorite editor, saves it, crontab then performs a sanity
check on it. If all is good it copies the file into the crontab
directory and notifies the daemon of the new crontab.
I think the current system works well...