I have seen manual wheelchairs folded on buses. The problem is when the person supposed holding the wheelchair decides to put it in the space in front of a wheelchair and then refuses to hold on to it. It is unsecured and can fall and hurt the person in the wheelchair that is tied down. Another downsided is that some decide to put it between seats and take up seats (as soon with double strollers do as well).
I refuse to allow one placed in front of my wheelchair for my own safety.