Writing is just like any other profession: you have to learn to master it. No one, NO ONE is born a great writer. It takes practice, practice, and schooling (whether it be paid or researched).
So taking classes, IMO, isn't a waste of money or time if one is seriously considering a career in writing.
Take the baseball example. You think the people playing Major Leagues never practice? Learn stuff? Go to places? (OK I know nothing about baseball so maybe it's a bad example for me to use).
Acting. Acting I'll use, because it's somewhat relevant to this kind of writing. Now I don't have any factual evidence here, but from what I've read (articles, interviews, etc) of other well-to-do or even average actors is they, at one point in time, have taken an acting class of some sort. Time. Money. Yet it equaled to some sort of success.
Practice can only get you so far if you lack the knowledge of really perfecting your craft. I believe a writer gets to a point in their writing where they don't move up, they continue on the same level of writing, and it's mediocre. Writing workshops, critique groups, classes, retreats, conferences, all different things to not only get connections and network, but to learn. To become a MASTER of your profession.
Now, if one isn't serious about this and it's just a hobby, than yeah, I can understand maybe saying it's a waste of time and money to find a workshops/class, etc (but do your research, not all are cracked up to be what they should be). But still...if it's just a hobby, why get so worked up if your stuff isn't selling or getting picked up?
All of the above is simply my opinion, of course, take it as you will.