I've often thought of these types of problems. I don't have any personal experience to share, just a few thoughts. Well, I have seen counselors and therapists, but not about poly.
She says she has never once seen it work and that it often destroys lives.
Dan Savage once said that he's never been invited to a 5 year poly anniversary and "good f***ing luck to you if you make it work," so your counselor is not alone in her opinion.
HOWEVER, I would like to point out that personal ignorance does not mean universal truth! Just because she has not done her research or homework doesn't mean she is right! Perhaps you can go with some ammunition!
Secondly, I disagree that "poly destroys lives" - to me that sounds like she is projecting her mono-normative values and making a baseless assertion. As you pointed out, there are many people who are unfit for any relationship let alone a poly relationship!
Again, she's not completely alone in her opinion. I've read many blogs and things written by people who felt like polyamory destroyed their marriage. Just do a quick Google search of "polyamory destroyed my marriage" and you'll see there is no shortage of anecdotes.
My personal theory is that if something outside of a marriage could 'destroy' it, then the marriage was probably doomed to begin with. In some cases, one partner wants to explore polyamory and goes wild, leaving the monogamous partner feeling abandoned and neglected. That is the fault of an irresponsible partner, not polyamory!
But then look at the other side. There's plenty of evidence and talk that polyamory and non-monogamy are really legit and healthy.
Anyways, since you like your counselor so much, have you considered asking her to read up on the subject? You could even bring her a magazine or a print-out. Or email her a link. I would make it clear that you really value her insight and that you would like her to become more familiar with your lifestyle.