'll interpret your question as referring to new equipment design vs modifications of existing vessels and piping (which becomes much more gray).
Legal? Depends on the jurisdiction. If ASME code is not a legal requirement then you are free to legally use any edition of the code or any other code at will. If they are legally mandated, then they may also mandate "current edition" or a specific one.
Clever? No. Would you be willing to 'splain to the judge that no new knowledge has been incorporated into the codes for 30 years and that the failure which might have been prevented by using more recent codes was just a fluke? If we as an industry knew everything there was to know about vessel and piping design, we could have written a pressure vessel and piping code back in 1900 (or whenever) and never have to revise based on experience and increased knowledge. Let's not forget about getting insurance coverage either.
Ethical? No. Would you honestly frequent a dentist or doctor who used 1970's equipment and medicines and had never attended any refresher training? So why would you expect your clients to want "less than current" technology/knowledge implemented in their design?
An interesting question sure to generate some discussion...
No rule requires that anyone have any standards. What do you do or make? What are you doing with the standards? Are you only dealing with ASME; or do you need API, ASTM, CGA, IEC, ISA, MSS, NFPA, etc.?
If you fabricate vessels you are in a different situation than someone who buys pressure gauges.
Many large engineering companies subscribe to standards via IHS, a standards provider under license to the various professional societies.
Among considerations, if a project kicks off in 2008, the codes and standards for the project should be those current at the date of the contract or kickoff. I keep many current standards as pdf files. However, as ASME and other clubs update hundreds (perhaps thousands) of standards every month it is not practical for every standard to be current.
