They can be desirable. If I want to normalize the group delay of a bunch of drum mics, I sure as hell not gonna use a BBD-Delay for it. But if I want a flavourful analog delay onna guitar track, why not.
"Am I having a stroke" comes to mind :) A bunch of drum mics? I'm no recording engineer, but I guess this is about mixing different signals from the same setup, with the signals having different delays (possibly due to filters, cables, or different types of mics)
Microphones at different distances receive a signal at different times. When combined, the signals are not phase aligned. This causes comb filtering where the combined waves reinforce and cancel each other.
Though that is sometimes desirable (e.g. comb filtering is important for spatial ambience such as reverb).
Some audio engineers find careful microphone placement solves many potential problems.
Drums are notoriously hard because modern recording techniques use many microphones on a kit.
