@janoteelem: If your problem was an inability to select sending addresses from the "From" field, and the action you described fixed it, that's good news.
But FWIW, I think most people are concerned with something quite different. In my case (and some others'): I have no problem freely selecting a sending address from that "From" list, whether in ML or any other version of Mail, or whether or not I've fiddled with SMTP settings. It's not an issue for me.
The problem with ML is the need to change the sending address in the first place (in situations described elsewhere in the thread). As with Lion and other versions of Mail, and every other email client I've ever seen, I expect the default sending address of a reply to be the same as the receiving address, without my having to confirm this and change it as necessary. ML Mail is the only email client I've seen that breaks this behavior.
I played around with SMTP server selection just now as you describe, and unfortunately, for me there's no effect on ML's wacky Reply behavior. Still a major bug (or inexplicably poor decision) on ML Mail's part.