As I mentioned before, there's a right and a wrong way to do partials.
I'll use db bench press for example since oldtimer1 mentioned his story above. Stopping short the way that kid did has no benefit and actually is detrimental to any kind of development.
I lower the db's to a point actually a bit lower than a bar would allow. I then press, but stop about 4/5 (or a tad less) of the way up, far short of lockout, and even before triceps take over the bulk of the load. Done in this manner, the pecs never get a rest due to overbearing assistance from secondary muscles.
Of course, there's going to be some involvement of ancillary muscles, being a compound movement. The idea is to minimize involvement of these muscles as much as you can to maximize the effort of the pecs. And, again, I would only recommend this strategy if one's goal is strictly hypertrophy.
Functional strength is an entirely other matter.