The problem here is that this world is filled with lots of people who don't know what they don't know.

To a new or otherwise uninformed cyclist riding 10mph or riding for an hour or two might be be pretty noteworthy.

Shifting gears? When they learned to ride xxx years ago, they didn't have any gears.

But how will they ever learn this if they don't go on a group ride and learn their shortcomings?

I guess if the ride leader just assumes that there will always be some people that need more attention and lowers his/her expectations, it will be a better experience for all in the long run. The ride leader gets to develop coaching skills, the new rider gets the help he/she might need, the group may develop some new riders.

I am not a fast rider myself but I do make time on occasion to work with new, slower, or just less experienced riders in the hopes of developing future riding partners.