There are two separate issues here
1) Garbling. This happens if multiple transponders respond to the same interrogation, e.g., with same slant range but different ground range, or when transponder replies overlap. This is an increasing issue because of
- more and more interrogators flying (TCAS)
- many ATC radars in Europe with overlapping ranges.
- more data being transmitted in the transponder reply (enhanced Mode S, ADS-B etc.)
S stands for "selective" - a standard Mode A/C transponder will reply to any interrogation for that mode, a Mode S transponder only to one that is directed at that individual transponder, using the 24-bit, which avoids garbling. A Mode S radar does infrequent "all calls" to find out who is around and then interrogates each transponder individually.
Or, put more simply, Mode S is a fix required to accomodate a much larger number of transponding aircraft. That is why universal Mode A/C is not sufficient and the authorities want mode S even for those aircraft that in effect will only ever transmit a 12-bit squawk and pressure altitude, and not only for the heavy/fast guys that transmit a lot more.
2) Screen clutter
That is up to the radar display software, can't comment here.
I am afraid the real debate is whether everyone should squawk or not. If the answer is yes, Mode S is necessary to avoid the ancient A/C technology breaking down in some locations. And while there are better technologies that could do the trick, Mode S is the de-facto incumbent in the airline world.
Maybe the debate is more usefully directed at how to minimise the impact. Do we have to pay silly mod fees for paperwork for safety related equipment?