My daughter is 6 (and a half, as she’ll proudly tell you) and at least 44 inches tall.
She is well Above the height limit for some of the rides we used rider switch on. (Splash, big thunder, etc)
In our experience, They never once measured her or asked why she didn’t want to ride. Just asked who was riding in the first party and who was riding in the second.
(Also for height reference, she did ride test track, mission space (green AND orange) flights of passage, dinosaur, and waited in line for tower of terror (just because she wanted to see the queue...), and she was never stopped to see if she was tall Enough for those - so I *think* most cast members recognize on sight that she’s tall enough to ride the 40-42 inch rides.
I think they’re pretty much using it as “here is a child who is too young to wait alone, and doesn’t want to ride, so child swap is an option. “