but if a vocalist is going to come in and either replace all the vocals, or do them all from scratch, most engineers would by far prefer to do them in a nice sounding room rather than a dead tiny booth.
It seems to me that engineers do it a certain way because it works. If recording in any room they wanted (possible for some) was better. It seems that they'd do it that way. Dead room plus effects is preferred probably because it's convenient and because it works.