Most states require a mechanical parking/emergency brake. If you use a line lock you need one that is closed with the current off or it will run your battery down. Also there is the issue of the brake rotor being warm when you apply the brake and as it cools and shrinks the brake will slip.
Wasn't there a guy who had a "Line Loc" parking brake in San Francisco? He parked it for the night and in the morning he realized that his battery had gone dead and his car was in the Bay.