What people SHOULD keep in mind, is that a GOOD engineer, is an equal partner, just as the musicians are, and should be considered and paid as an equal to any of the band members. ..That being said, you can't expect an engineer to 'fix' a bad delivery of the music...though some can do wonders!
As far as, you can't understand the words', that has a LOT to do with the vocalists!!..and here is the rule, etched in stone: "The human voice is the only instrument on the planet that pronounces words, so for God's sake PRONOUNCE THE WORDS, and don't HIDE behind 'style'"...Hold only the vowels, and bite off the consonants!
I have insisted through the years, that musicians I play with should learn SOUND(engineering)...because when it's all said and done, WE ALL are working with SOUND!!...no matter how much 'pride' you take in your particular instrument, the bottom line is THE SOUND..and it is up to the individual player to deliver a CLEAN performance with expression...you can't blame a sound engineers to turn crap into a stellar performance...but as the expression goes, "A bad mechanic always blames his tools!"

GfS