je.f
At London ACC Swanwick your mode C height readout disappears completely from the SSR label during high rates of descent. I might not see the rapid descent rate, and I would be left with a callsign label and no visual cue to any problem. I think a "fix" is under way after years of asking. A 7700 will draw my attention....and everyone elses. It stops other units and sectors calling me because they know I'm busy.
As a side note it is worth mentioning that the labels of military aircraft with a high rate of climb and descent, and which operate in close vertical proximity to civil traffic, are likewise affected.
I'm only a controller, so I assume the iron brain thinks "gosh! that can't be right.....I'll switch that off"
Imagine you are over southern England at high level; below you may be several layers of airspace with each level banding group controlled by a different sector or agency......select 7700 and everyone is alerted simultaneously. Official "book" answers may differ.....I only know what works best in real life.