I guess it depends on whether you want to update the trim for your fixture either now or in the future. i'm assuming, since your valve is 40 yrs old it has 2 screws which hold the finished plate onto the valve and both of these screws are located at the bottom of the finished plate. All newer Moen valves have the screws located at 2 o'clock & 7 o'clock. There will come a time, NOW, when you will find it difficult, if not impossible, to find special finish trim for a 40 yr -old valve. They just don't make the trim for older valves anymore. Also, it's been my experience that the replacement valve body doesn't usually fail ( it's just a forged piece of brass ). Typically it's the mechanical internal parts of any valve that fail. If you replace a faucet with let's say a new Moen and you register that faucet with Moen, I can assure you that the manufacturer will stand behind their Life-Time / Life-Shine Warranty on that product. I have seen Moen send out brand new complete faucets to satisfy homeowners.
Sooooo, very long story short, replace the valve-body if ya want to modernize. Keep the old valve body if ya want to remain status-quo.