I am with you, nothing beats seeing a steam locomotive operate and a live locomotive generates way more interest than a dead one. That said, in a museum environment not all of the should run, some should be left un molested as a record of how the railroads did things way back when. Along with being educational, an undisturbed locomotive can be a big help when it comes to figuring out how the one that arrived in pieces goes back together.
As to your original question, there is nothing really wrong with stealing parts off of one locomotive to keep another one running, the D&S and C&TS do it all the time. The problem, again in a museum environment, comes when you do not put the one you stole the parts from back together, especially when the one you stole the parts from is unique and arrived at the museum intact and as it was when last used by a railroad.
D&RGW 638 is the only standard gauge Rio Grande locomotive on the face of the earth and it was brought to the CRRM to preserve that status. 638 was very, very worn out when it came to Golden but it was COMPLTETE and remained that way for over 50 years on display. A museum should not cut corners on an operation restoration by ruining the historic fabric of another object in its collection. If parts were taken from 638, they should have been traded (ie take the good check valve off of 638 but put 491's crapped out back in its place so 638 remains intact and complete) Canabalizing one asset to operate another is not a sustainable course of action for a museum.
To put in in aircraft terms, there is an untouched, original Battle of Britain RAF Spitfire hanging from the ceiling of the Museum of Science and Industry in Chicago. It was crated up and sent to the US in 1941 or so as a thank you. It wears is original paint and is exactly as it was when it last saw combat. Would anyone be OK with me taking parts off of that Spitfire so I could make some other airframe fly again? NO, the correct answer is to make the parts you need and leave the priceless original object alone.
D&RGW 638 is even more rare than an untouched Battle of Britain combat veteran aircraft, it should not be used as a shortcut parts supply so that 491 can pull a longer train around the museum's loop of track.
(As a side note, this is why C&S 60 in Idaho Springs should be left alone, it is the only C&S locomotive left that is still entirely as it was on the C&S)
Jason Midyette