Exactly. I don't do much of the guided tours thing, but the last person in a group should be someone who is capable of spotting and dealing with trouble.

What's the point of a group if the members can't take care of each other?

Not to mention the bad effect on morale if the weakest members ALWAYS fall WAY behind everyone else (quite often, the slowest members will also be the one least capable of taking care of themselves). Having someone more capable back there really boost the morale of both those slow goers as well as the fast members.

Personally, I fall behind if no one else assumes the role. Besides, it's quite nice to shufle along at a sedatary pace, chatting and having a good time.