If mastering the use of a mag release is OK why not master the use of a slide release at the same time? It's definately faster than 'sling shotting' the slide. (Speed is a tactic!)
Here's the issue with using the slide lock. On most pistols the slide lock is a small surface. In the case of Glocks its a very small surface and is difficult at the best of times to reach. This is no accident, it was designed that way. The fine vs. gross motor skill debate isn't one, its fact.
The stress experienced during a match(IDPA) while high obviously isn't the same level you'll experience during a gunfight or any other life threatening situation.
Ones ability to operate the slide LOCK under stress can be improved or enhanced with plenty of practice, there are those who are quite proficient with it. However, the use of the slide lock does not apply to most situations under most circumstances. The use of the "sling shot" method or rather grasping the slide does. Use of the slide allows the recoil spring the opportunity to achieve full compression. It has been said a full 90% of the springs potential force is generated in the last 10% of compression. This last 10% is not achieved when "dropping" the slide with the slide lock.
Ask yourself this. Does the use of the slide lock work with wet hands, muddy hands, bloody hands, gloved hands? Does the use of the slide lock work with your left hand? Is it easier to locate the slide or the slide lock in the dark? Do all pistols have a slide lock to use? Does it take more effort to force the slide lock down against the recoil spring or retract the slide using your hand/arm/upper body? Can even the smallest shooter operate the slide lock or the slide? Why train to use the slide lock when it only solves one loading problem?
Here's your answers.
Slide locks are difficult to locate and operate with wet, muddy, bloody and gloved hands.
Operating the slide lock with your left hand(as in pistol is in your left hand) doesn't work.
Locating the slide will be much easier than locating the slide lock in the dark. The slide comprises approximately 50% of the external area of a pistol. Your slide lock accounts for??
Not all pistols have slide locks, so why learn a method that doesn't apply to most if not all pistols?
Retracting the slide directly counteracts the forward force of the recoil spring. Using the slide lock does not, you are simply using force to slide the lock down and out of the way with the use of your finger only.
The elderly(and the young!), physically disabled or the petite in society cannot effectively operate the slide lock. If those above cannot operate the slide then using a semi auto is a non issue. Why complicate the manual of arms for those who struggle to begin with?
The use of the slide lock solves only one possible scenario. Slide locked to the rear and an empty magazine. The standard method of loading(administratively) is to insert a magazine then cycle the slide. The standard IA drill for a failure to fire is to TAP the magazine and RACK the slide. The same drill is used for two different situations, administrative loading and your IA drill. If you're loading your pistol from slide lock(known as an emergency reload) the only difference between your administrative load and the emergency reload is the level of urgency and the position of the slide. Does the position of the slide make a difference if you always use the slide/slingshot method of reloading. Answer, no it doesn't. Does the position of the slide matter if you use the slide lock. Answer, yes it does.
Why train yourself in a technique that is only applicable to one specific scenario. Why not minimize your manual of arms by using the same technique(or as few as possible) to accomplish many tasks.
Loading your pistol involves two movements. Inserting the magazine and cycling the slide. If you cycle the slide every time you insert a magazine you will never be without a loaded pistol. Regardless of slide position you cycle the action whenever a loaded magazine is inserted. That two step process works for all reloads and it works as your IA drill on a failure to fire(with the exception of not inserting the magazine but tapping it to ensure it is seated).
On a final note. If you shoot IDPA and are concerned with speed rather than accuracy and effective tactics, then you're in the wrong discipline. IDPA was designed around ones ability and RIGHT to carry for self defense(of course this only applies in a free country, not Canada). Defensive shooting isn't about scores or fast times.
TDC