pretty stupid suggestion to do that but in most cases they just do it for prevention, ive rolled and snapped many ligaments in my ankles as im a basketball player never had it in a cast at all, if they do put it in plaster will become quite weak while in there...from my point of view if its not actually broken it shouldnt go into plaster..you will be suprised how far R.I.C.E goes!!!if you do it properly.Your cartilage prob would have stretched then and because you keep walking on it is just continually reaggrevating it so thats prob why he wants you in plaster
With the knee it does sound like cartilage problem and i would refrain for bending down at this point and like others say impact sports...likely hood with the amount you do you will need it shaving and possible microscopic tear of one of your ligaments so keeping it microscopic is key till you get it sorted...you both will be back to norm in few months dependant on your physio