I also prefer to play Diablo solo, but the group bonuses aren't what I feel are the problem. It's everything else you get from grouping. Want to do bounties? Or would you prefer to do them 4x as fast? Want to farm machines? Or farm machines 3-4x as fast? Want to kill ubers? Or kill them faster and spend 1/4 the machines.
I don't mind multiplayer giving a % bonus to things, I DO mind it being a ridiculous requirement to not feel like you're wasting your time. I feel like a buffoon doing bounties solo, it's simply a stupid way to do it. And it shouldn't be.