This technique makes it easier to coordinate the hotspot of the light and the sights of the weapon, in my opinion.
It is also a very free technique, allowing you to quickly adjust the orientation of the light if you need to do so.
It's very easy to transition between this technique and the FBI technique.
The neck index technique is also easy to get wrong.
The most common failing I see is folks who end up improperly indexing the light so that it ends up shining on the rear of the handgun, which partially backlights the shooter AND makes it nigh unto impossible to see the sights properly… especially if they are using a face melter like we discussed in the gear section.
This is also a technique that limits you to one handed shooting.
Another mentioned weakness of this technique is that it directs incoming fire at your head and upper chest area if someone shoots accurately at the light.