Well, seeing as how Red plans to add seeds & grass blocks I've been thinking about people's strong desire to not have grass grow back. I say that it should in that bit of Minecraft style. It should grow back, but more in weeds and such forcing you to weed them out to replace with grass seeds. It should also be biome specific, whatever that may mean to weeds and growth rate.
For those worried about having their dirt paths grown back on may want to suggest a spray bottle to kill plants from spawning. Basically have a spray bottle object, fill it with whatever fruit+vegi liquid you can, then spray the cleared grass from it. With that, the grass should stay clear of both grass & weeds. Even those tall grasses & biome-specific plants...Or so I'm suggestion. If you want the grass to grow then spray water then plant the grass seed and watch it gradually grow......
1) Grass should grow back, weeds should grow with higher chance, and biome specific rates for both.
2) Having a spray bottle to spray fruits & veggie mix (or some other more appropriate mix) to keep the grass from growing at all. A one-time use. To clear just spray water.....
Maybe this is too much, or too simple. I'm simply trying to suggestion that would please everybody, thus me recommending this...I really do want to see grass reclaiming disturbed areas but people keep stressing that they don't want their pathways to disappear. I'm simply hoping what I suggested made sense and that it's a nice middle-ground to work with.