It depends... if someone is trying to use every muscle involved with say bench press (chest, shoulders, triceps, etc...) than locking out would be of some merit... although I hardly ever do a complete lockout on EVERY rep simply because that would be pretty tough on the elbow joints. If someone is trying to emphasize more chest on the flat bench, not locking out keeps the tension on the target muscle group (chest) so you work it harder. Locking out moves the tension from the chest to the triceps and shoulders.