The issue with the Groove is not that you can't take one and find a use for it. The issue is that some Grooves are overstable, some are flippy, some have glide, some drop like a brick, etc. That's not terribly uncommon, but in most discs that are inconsistent you can find a tell; the domey ones suck or the flat ones suck or the ones with the flatter wing fly less stable or look at the PLH...SOMETHING gives you a clue as to what the disc is going to fly like to help a player with some experience find the one that will fit his needs. The Groove doesn't. Two discs that look identical will fly differently. You just have to buy one and hope it's going to fly the way you want it to. Nobody wants to do that. It's that "every one is a unique snowflake of suck" quality that people hate.
If you have one that you like, or you like buying discs that you have no way of knowing how they will fly, the Groove is a fine disc. I'm just not going to throw my money away like that when there are other discs that I can buy and I do know what they will fly like.