Why are suckers bad

I think the red cane is a sucker. I haven't dug to see exactly where it comes out but seeing where the scion is and angle of the branch, I'm guessing it's below the graft.

Anyway, why is this bad? I'm, perhaps misguidedly, excited to see what sort of flowers might come from it. I guess I'm more of an 'explorer' so, in general, I just let everything grow to see if I like it.