During a recent trip to WDW, I saw four adults walking in a park (I forgot which one), who each had a cane with the small attached seat. I told them I didn't know that you could bring those in the parks, and they said it was no problem. They obviously passed through security without any issues. For what it's worth, I would characterize the group as "seniors."
It can be difficult for those who have back, knee, and other orthopedic issues to stand for long during parades and fireworks, so I imagine being able to use one of those canes can be quite useful.