I too, have off-loaded barely ambulatory or otherwise visibly disabled pax who were traveling without a helper.
It is primarily a SAFETY issue...one who can barely walk could fall down during an evacuation thus endangering others...also as mentioned above, the crew are not nurses or baby-sitters...
When flying commuters years ago, I even offered to take a relative Free of Charge to assist during flight, and then bring her back on our return leg...she was to busy, so I guess her day got screwed up because Dad had to stay with her...
She decided to wait 4 hours for the next flight...Guess what?...That's right...me again...