I think I agree and disagree with some of the points being made here.

A 20mph pacelining group on a MUP is not only dangerous, but also against the rules, since most have a 15mph limit. But this is somewhat of an extreme example, and doesn't do any good to beat a dead horse.

What about a 15mph paceline group? They are in every right to be on the MUP, though whether this is the best place to paceline is another story. I don't agree with the "duty to be extra careful" if you have the ability to do more damage, because it goes both ways. Why shouldn't you be more careful if you have the ability to take more damage? While cyclist should be aware that they will need to slow down to pass other people, everyone else should also be aware that there will be cyclists zipping past them at 15mph. Letting a 5 year old freely travel along the MUP is an inherent risk not just for the child, but for everyone else.

In my area, there are more cases of ill-informed families with kids running all over the place than cyclists blasting over 20mph. Some are just in complete panic when they hear "on the left" behind them, and they just freeze, taking up the entire width of the road. I think there needs to be more emphasis that a MUP is not the same as a sidewalk, and also not the same as a road.