Another reason you might see yellow bands in the main queue could be group size, with anxiety and other people being in a larger group e.g 5 or more people some people may struggle more than they may let on to in silence, simply because they dont want to break into 2 groups ect.
Other reasons include not wanting to ride on the back row for example: if a yellow band holder wants to specifically ride on the front they are forced to hide the band and queue.
Again I think it depends queue to queue in relation to queue legnth, path width and if like the saw queue you can feel trapped at times.
In terms of day to day operations queuing at the exit can be a big deal as so many people make comments at rides exits about it being exit only ect
In summary every band is issued for different reasons as such each individual person with a band maybe affected differently in terms of there physical and mental limitations, no one member of staff customer or otherwise can truely know who is capable of what , some people abuse the system but most dont,