My problem with Calvin is just this: We never met him.
I don't care for original characters in fan fiction (I love them in original fiction, LOL), and I liked the show so my preference is for the characters and settings we knew.
It's not one of the things I have a problem with, and I was never committed to Drew and Emmett until I wrote the story my friend Travis requested. But I wouldn't want to read about Calvin and Emmett because it would be like Emmett with an OC (even though I know technically he's a canon character).
If we had gotten to know Calvin, I would feel differently (assuming he and Emmett were good together, of course).