For the purpose of provoking discussion, I will opine that most people's use of high iso film is no more than a phobia about exposing at a setting less than suggested by a general overall meter reading. They refuse to accept that 2-3 stops underexposure will frequently only lose shadows, which may be beneficial in conveying the impression that the scene was dark, and probably still give good midtones which are the important element.
Internet lore seems to have blindly convinced many photographers than any exposure less than indicated by an overall meter reading will cause that frame to be totally blank, the rest of the roll to be unusable, probably all shots you make for days will be unusable and will increase your potential for ED.
Many also seem to think that there are magic developers that will make an iso 400 film into a 1,600 or 3,200 film when the reality is the size of the silver grains is the only factor.
Too many shooting film and using hybrid workflows suffer from grainy images and blown highlights from extended development of their film without realizing how much can be simply accomplished with image editing software. Do they not realize how digital cameras get those incredibly high iso settings?
An example of an iso 400 35mm film (either Neopan 400 or HP5+, have to look at the actual negs to tell which) simply underexposed and developed normally is this image shot at the Ground Zero Blues Club in Clarksdale MS (sorry for reusing, but it makes the point)