I myself would be one of the people that argue for items in Smash Bros. and against blue shells in Mario Kart, I just think it's funny that the two games get people to come to the exact opposite opinions on this issue.
Again, I think it's more that most people agree that items in Smash Bros do require skill to use, as do most of the items in Mario Kart. But if the Blue Shell were in Smash Bros, it'd essentially be a one-hit-kill item that requires no aiming, timing, or proximity to use, and that can't be countered, blocked, or dodged, I think most people would hate it.
Oh, and I do agree that your observation is valid and fairly amusing, but I do feel the need to analyze the differences between the situations.