I am not sure what MRAW and SRAM are intended for, maybe special applications where it's important to keep the 12 bits at smaller resolution and be able to store thousands of shots. I do not know - I am most curious as of why one wants to use MRAW?
I shoot a 5d3, and always shoot mRAW during getting ready photos and receptions at weddings - the only times that couples are really going to blow their pics up is during the portraits, so that's when I shoot full RAW. While many gripe about the 5d3 being "only" 22 mp, that is still far too big for me to want to deal with for an entire wedding. Managing that much data for 30+ weddings throughout the season is especially difficult when you factor in backups and archives, so, imho, that is precisely what the smaller raw files are for - people who shoot massive amounts of pics but still need/want the extra flexibility of raw.
And on the IQ front - I honestly haven't noticed any difference between when I shoot full or m raw, but I am by no means a pixel peeper. They both seem to have similar flexibility in LR, but I've never really done a true test. I'd say that even if the IQ was, say, 5-10% worse, it would still be worth it with the size savings.