If all you have ever experienced is garbage, you become accustomed to it and act accordingly. I used to own a 1950's car that had laughable brakes, but you quickly adapted your driving style to suit the brakes and I never had any problems, but could I keep up with modern day traffic? absolutely not! Did it matter to me? No, because I would never dream of using that car when I was in a rush or needed to get somewhere in a hurry. Likewise, suicide levers can be used on bikes but should not be confused with proper functional brakes that can be depended on in accordance with that which is defined by modern standards of performance.
Back in the 70's and 80's when I worked in a number of bike shops, we removed far more of these extension levers than we were ever asked to install (by a margin of at least 10 removals for every one installation), so this would seem to indicate that the general consensus of those who did use them found them to be inadequate and not worthwhile.
PS: Ex Pres is correct about "stretching", what normally does occur is that the strands of the cable "bed" in with use, making for a more tightly wound cable over time, but the individual strands do not increase in length.