I read endless letters in railway magazines of the 1970s and early 80s ridiculing suggestions that diesels should be preserved, because don't have the character of steam and don't belong on heritage lines. Now diesel galas are an intrinsic part of many heritage railways. Literally the same views resurface every 20 or so years when another generation of rollingstock is retired.
The job of a heritage railway is to preserve railway heritage. While the Pacers ended up doing things they were not designed to do for about twice their intended life, they are nevertheless an important part of railway history which needs to be preserved. Ergo, they belong on heritage railways.
The National Tramway Museum at Crich and the London Bus Museum at Brooklands have preserved vehicles of similar vintage to the Pacers (and even buses built this century are preserved) for a reason - they are a part of history, no matter how much some grumbled about them in regular service.