I have seen producery behavior be destructive. I have never listened to a record and thought, "that could use a little more producing." I have witnessed greatness that required no external input. I have an innate suspicion of someone telling an artist what his art ought to be like. I have an in-built respect for the art of my clients and the clients themselves.
I don't think it is possible for me to assume that I am mistaken on all of the above, and a few hit records being made by bands with producers won't change my mind. How many flops have these producers produced-up anyway? Don't those flops prove me right? No, they don't, but they indicate that producing is not the answer. Don't play the results. Play the process.
I have no quibble with a band inviting someone into their fold as a co-operating partner. I think such instances of true collaboration are probably pretty rare, and recording folks who pretend their "How about some tambourine?" constitutes "collaboration" are mistaken.
I know many of my peers do more than that, going so far as to re-structure songs, write parts, sing and play guitar, etc. for bands that hire them. Two things strike me as ridiculous about this:
1) If he's so good at all this stuff, why doesn't he just make a record and be famous?
2) If a band is so dissatisfied with their music that they need all this stuff on it to be content, what the hell did they start with, and why did anyone think it was worth recording? "Boy, this material sure is clumsy and weak. Fantastic! Let's get into the studio right away so we can get rid of it and record something else!"