The pull request says
For example with negative speed + spherical shape you can make very nice effects.
I'd like to see an example of exactly what this looks like, because I'm having difficulty imagining it.
As for other use cases, attacks that shoot rockets might look interesting with acceleration. I think for really long range attacks, though (the thing I have in mind is the nuclear missile that I previously implemented in Military, which was able to target anywhere on the map),
we'd need a way to specify the maximum speed, so that the projectile won't accelerate too fast. Rather, it'd start slow and eventually reach a max speed (very much like a real missile).