My experience with stock, undiluted, D-23 is rather large and spread over several years.
When I was a young teen in the 40's i worked in the darkroom of a photography department. Our last task each day was to prepare all the roll film taken in that day for development. The film was removed from the paper backing and, along with the 35mm, hung from a clip on a large rack. A 2 oz weight was clipped to the bottom of each roll. the film was then lowered into the large vat of D-23 and left overnight to develop to completion. In the morning processing was completed. The D-23 was never changed as long as I worked there, only replenished.
Fast forward to the 1990's when I was teaching photography at the local community college. One of the methods of film development available to LF students was nitrogen burst. The developer was once again undiluted D-23. the solution was brought backup to the proper level with more developer. This was changed at the beginning of each new semester. I brought the used developer home and used it to develop negatives made of flat subjects in very flat light, once again to total development.
Currently I am on occasion using a batch of D-23 first mixed in 2012. It does not get replenished.
I just finished two series of tests which utilized 30 sheets of 8x10 film. It was developed in the D-23 by inspection in order to assure the film curve I desired for the tests.
Obviously in each of these situations the developer was, and is, far cheaper than diluting it for one time use. No, I do not replenish, or extend development times, but I am very careful to not contaminate it with other chemicals, particularly carbonates.