The majority of training-intervention studies in sports are crap. The sheer volume of them doesn't make the ensuing conclusions any more reliable. As a matter of fact, I think we would be able to reach much more reliable conclusions with one-tenth the studies but with higher average quality. My opinion, based on the literature and on personal coaching experience, is that it can vary a lot from individual to individual. Studies show a fairly strong general connection, but I've seen high variability: some athletes' vertical will increase a lot after strength training, some others' not so much, yet others none at all. The responsiveness may have to do with a number of things (like the speed-strength curves of the athlete and his elastic abilities). Not to mention how some people are pretty damn unresponsive to exercise in general. You see, I don't find that argument convincing. If we consider a sport like basketball, for example, there is no doubt that individual success is multi-factorial: not only has it to do with different athletic abilities (jumping, linear acceleration, multi-directional movement ability, etc.), but it hugely depends on technical, mental, emotional and situational/environmental factors as well - to a point that cannot be overstated. Tons of players with far inferior athletic qualities than others have far greater professional success. It is so multi-factorial that I would expect it to be virtually impossible to isolate and test any one narrow parameter (for example, the same exact player may have an entirely different level of performance from season to season depending on which team he lands in, which league he plays in, which coach he works under, the roster of the team, etc.). As a matter of fact, I would argue that, if a player manages to increase their vertical in the off-season, it would be misguided to expect that would have any sort of clear correlation with success in the upcoming season. I would expect it would correlate over a number of athletes over their entire career, which is something that cross-sectional/correlational data do agree with. Having said all that, you would be hard pressed to find a basketball player that wouldn't want a bigger vertical, or a basketball coach that wouldn't slightly value a player with a bigger vertical. One could use the literature to argue that this is due to the effect of bias and that there is no good evidence a bigger vertical positively affects basketball performance, but I would personally consider that person a dumbass who engages in mental masturbation. I've actually seen less/no proof that doing stuff like "agility training" will have a transfer on sports performance. Which isn't to say it won't, because I would expect that to be an even harder hypothesis to prove.