In many parts of the world, some of them very heavy usage areas, there is no primar radar cover, so if the transponder goes off, no one in the control centre can see the aircraft!
It may be visible to the military, and sometimes, if the centre concerned has primary radar, they will still get a return, but it's far less common that most people realise, and I'm not being alarmist here.
In many cases, that's why the MEL requires 2 transponders on the aircraft, so that if one fails, the flight can continue without problem.
So, transponder that can't be turned off may be a good idea, but then again, if there's a serious electrical problem with said transponder, how do we then disable it? Pull the entire flight deck power? I don't think so.
Also, the Txponder needs to be off when on the ground to avoid horrendous problems for ATC with conflict alerts due to separation problems. Thare are other problems, but these 2 spring to mind most rapidly.
As always, quick fixes may not necessarily be the answer.