as a soldier in a scout dominated meta losing a rocket to recover 40 health in a best-case scenario (cuz you probably used 1 or 2 rockets to jump in) just isnt worth it. the extra rocket could probably be used to kill the person you're fighting before you needed the health from the black box, or to jump away to a health pack and get 100 health without needing to hit ur shots. +60 if you hit 3 90 damage shots, or +100 for using 1 rocket to jump away and still having 3 to fight if the dude chases you. i'd take the guaranteed 100 health any day
maybe back in the day it was much better situationally, say, being able to shoot one rocket across the map and hitting a few people and collecting 45 health for free, but nowadays when you can only get a max 20 health back per rocket i dont think it's worth it.
as rain pointed out though, i imagine it could be good in highlander with a banner, but i dont play highlander so i dont actually know