I honestly don't know the scientific literature regarding plyometrics, but I don't think most traceurs should really bother with them in GPP. Jumping is a skill and you're better off practicing in ways that will develop cognitive functioning in parkour specific situations. Max effort precisions, bounding and striding precisions as well as other various parkour related skills like gettiing max distance after vaulting are all more useful to someone who wants to get "better" at parkour.
Parkour is really heavily skill oriented (unlike track and field for example), so I'd rather spend my time developing better visual/proprioceptive strategies, anticipation, and body control skills, than add plyometrics to my GPP (alongside strength training). Just seems like there wouldn't be too significant a difference in performance and I'd be better off overall because I would have practiced my sport-specific skills more.
Most traceurs? No, you're right. Most traceurs are weak as hell. Power can't come without strength and neither can speed. But most traceurs are also very linear, so agility drills
will have a significant impact on performance. Being able to function at a full sprint is going to impact performance. Etc. etc.
I don't necessarily disagree with you, but I think plyo training in conjunction with true strength work
and Parkour-specific training is going to be the route for your advanced athletes. In a lot of cases, advanced traceurs have pretty much tapped out on what technical training is actually going to offer them as far as performance goes, and that's where traditional strength and conditioning come into play.