My theory is that they don't make a big enough size recirculating blow-off valve for a reasonable cost, the SC bypass is about 45mm and most turbo valve's are 25mm, it might cause idle issues and from tinkering around with them, they seem to open slower which could cause a lean condition in between shifts.
Your also constantly trying to figure out how much spring tension is required since a 15lb spring may not equate to holding 15lbs of boost, there also seems to be a reliability issue resulting in a weak seal but on the flip they do seal better when adjusted correctly but noting that some valves create the seal on inside (the opposite of an engine valve seal) this makes it that much harder to hold a lot of boost, however some valve have an inverted style diaphragm which would make the seal look like an engine valve, a better seal, they may have that information posed on a Greddy website.
The SC valve, from what i've seen, creates the seal on the wall it is enclosed within, from experience I took the valve out and poured some water on them, to check for leaks, a few drops did seep through, however, if you apply a small finger pressure to the pressure side of the plate, there is no seepage as that would be a realistic condition if 10-15 lbs air pressure were acting on the inside and vacuum on the outside.
If you can fork out an extra $150 on a high quality turbo style valve then I don't see any reason why it can't be used. I opted for the remote Eaton bypass for my next setup.
My 02