Chuck,
MESC is a standard Shell classification criteria. And this is stipulated on the agreement between Shell and its vendors (read: distributors).
Even though I also quite agree that this is a standard specification (some say commodity valves), nonetheless Shell have reason why only qualified vendors are the preferred ones and why the valve should comply with its stringent specification.
This valves among other requirements must be Charpy tested at -50 degree Celcius at 50 Joule. And also have extended Bonnet.
I would be surprise shall you submit your quotation to random producer (which may consist let say less than 10 Valves), they would comply to it.
My experience is that the valve would be somehow deviated from one or several points from its spec. Thus, subjected for rejection by Inspection department.
There are many reason why the specification is there, and often considered as not important points by random manufacturer. Only Valve Engineer on the Plant who deal with it on daily basis who have sense of the requirements (especially at Complex refinery).
Suggestion is to consult with Shell distributors or Shell personnel itself.
Regards,