Good question. I've considered having this option set as default but have two reasons why it's not yet. First is I don't know how prevalent this problem is. I have bought some models with fbx files and couple models had few $AssimpFbx$ ($fbx for short from now on) bones generated but not in the extent which generated near 200 like some mixamo models.
Is it common? I don't know the answer.
Second is while it's it's common to have multiple $fbx nodes per bone in models, I have yet to see multiple $fbx nodes per bone articulated in animation. I haven't sampled enough mixamo animation files to know if there exists such case. If they do exist then complexity of extrapolating animation would became significantly more difficult; something that would require testing.
If anyone finds an fbx model that cannot be animated using this option, whether it's mixamo model or your own, I'd like to get my hands on it for testing.