Sorry to be blunt (end of a 4 day Euro tour as crew)

Aircraft accelerates...bag would slide out from said empty seat
Aircraft drops suddenyl after take-off, bag his you in face or other poor pax,
OR (god forbid)
Aircraft has an emergency...bag possibly now around our feet area bocking bracing position and any exit from your seat area.

If stowed correctly your legs will stop it from becoming dislodged...tada!