As an example, I have the new reservoir and last night while I was setting the valve lash on my car I accidentally pulled the line that feeds the clutch from the top of the hydraulic clutch master cylinder as I was installing the spark plug into the #8 cylinder. The wrench caught the tube and pulled it up (it's a pretty tight fit in there, had to use a universal joint on the extension). Luckily it pulled it out and I heard the sound of liquid hitting the floor.
Which of course lead to a much longer project as I had to jack the car up and work in a very confined area to remove the master cylinder and re-fit the feeder line into it
The point being, that the fluid drained down to that upper connection and then stopped. I still had my brakes. If it had only partially pulled out and created a small leak that I did not notice while driving, I still would have had brakes.
It is not advisable to run on a reservoir that does not separate the brakes for the clutch and also separates the front brakes from the rear brakes. A warning light does no good if you have a break in a line and all of the fluid runs out in a matter of seconds
Having separate reservoirs, either as distinct cans, or as a unit with seperaters for the front and rear brakes is a must for safety concerns. Many race organizations require them.