See, if we were talking about Mega Man, I'd agree with you. The difference is that Mario is less about enemies and more about environments, so a character who does not affect your movement doesn't really contribute anything to your gameplay experience. You and your friends can make NSMBWii as cooperative or competitive as you want, and the interaction encourages you to acknowledge and coordinate with your friends rather than proceed oblivious to them. You read them, you help them, and yes, you even screw them over, gauging for yourself where that line is between friendly pranking and sheer dickery.
My oldest brother calls it "divorce mode", but I expect little from a man who refused to play 2-player SMB3 because he didn't want to share Toad houses. Ultimately, I see people who complain about bumping as people who don't want to be troubled by other human beings, and they really should be playing single-player. Or an online game where you bear no accountability for how you treat others.
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