Changing your transponder code to 7700 in an Emergency Descent is all very well, but the priority with a transponder is to turn it to TRAFFIC ADVISORY(TA) ONLY .
Imagine the excitement generated by descending to FL100 (or the MSA) in IMC with a TCAS "climb,climb"! Do you stop your descent or do you ignore the TCAS? What will the other recipients of the RA do? If you're in a busy environment how many a/c will then be carrying out TCAS manoeuvers? Then at sometime (when you can get a word in!) you declare a MAYDAY to go along with 7700!! If you manage to survive then it's the controler who dies from a heart attack!
As a pilot, if you have the presence of mind to think about the transponder during an Emergency Descent then the priority must be to select TA ONLY. You can then discuss your actions in denser air!!