I might be wrong about this, but the grass uses the instancing system to merge meshes and reduce drawcalls, from my experience you can use super high densities without affecting performance, as long as you keep sub materials down and shader complexity low it will run just fine. I did comparison tests with High density and Low polycount vs Vice Versa. Both have equal performance/drawcalls due to merging. There probably is some advantage to having a higher density static mesh, but it’s not that significant from what I can see, I’m definitely not making huge wins in performance.
And this is the main issue: Putting more grass blades into a static mesh and spreading it over a slightly larger area (otherwise it looks too patchy) reduces visual variety and leads to floating geometry. For photorealistic scenes this is a big disadvantage. You can see this in my test scene attached, the more density I add to the static mesh, the worse the floating geometry on sculpted landscape.
I believe users should be allowed to push the engine to the limits. We should be free to use huge values and high densities.
You never know what good can come from unlocking a parameter like this.
Why restrict the users with arbitrary limitations.
At the moment I now have to make the choice between patchy grass (a mesh with a smaller surface area has less floaters) or floating grass (caused by larger surface area). The grass system was perfectly adequate before this limitation was introduced.