Some people (I think its safe to say MOST on this forum) believe the lock can engage and lock up the gun when you need it most through various factors, ie recoil, etc.
Personnaly I dont really care either way, I never use mine, and it has given me zero reason to believe it would fail, even after a session of about 75 125 gr .357 JHP through my 360.
Like I said, some really hate it, to me, its just more parts to have to deal with when doing detail cleaning with the sideplate removed.