This sounds stupid on first glance, but in practice this would be more useful than I can easily enunciate. Anyone who's ever dug containment tunnels in the surface layer knows just how irritating grass is, and how equally irritating it is to hit each block with a pickaxe once, carefully, while the stuff is growing back, and hope to god you're not using an endgame pickaxe or you'll just end up annihilating the dirt anyway. I'd have tons of use for this. Support.
Though for the base-block quandary, I'd just wish that grass wouldn't grow in spaces between two blocks, if the block doesn't 'mesh' with the other block. Because so many nice building materials don't mesh.
Edit: However, I don't think I'd want them to be dropped from shadow orbs. They drop combat gear or a light pet, which are useful at that tier of the game - this would be a bit of a letdown for most players.
This is a bit of an errant suggestion, but I'd make the recipe for this a Staff of Regrowth (which can easily be found before the end of pre-hardmode) and some neat number of ebon/crimstone blocks. 20/50/100.