Leaving auto ISO set with manual is kind of nice, in that it makes your guess correct by adjusting the ISO up or down to allow for error. If you are going purist manual with one set ISO, though, or practicing guessing exposure, you wouldn't want this.
I like it, because I've gone meterless and guessed exposure long enough.
I actually usually use aperture preferred auto most of the time with my manual focus Nikkors. Ten years ago if someone had told me I would be doing that, I would have said they were crezy.