Hmmm ... I tend to agree with you, MM ... to a point.
I do think that a model can certainly "try" poses in front of the mirror ... for practice and to learn. Pratice does help, some. I think that they need to do a few things when working with either a book about posing or just copying what they see in magazines because mimicing what they see in a book alone won't help much.
1. They must practice .... a lot .... hours and hours.
2. They must really try to be objective about how each pose looks on their body ... what flatters them personally, what doesn't and commit it all to memory.
3. They have to get good enought to move in and out of those various poses comfortably (lots and lots of practice) and be able to do it without looking in a mirror (I call it "body memory" ... because they won't have a mirror at the shoot.
4. They need to learn the difference between editorial/fashion and more commercial types of poses/movements ... because it's totally different. And make sure that they understand what the photographer is needing ... so they can adjust to suit.
4. It's also very important to learn how to use their face, eyes, mouth, etc. to get various effects and moods. And not to forget about what to do with their hands and feet, either.
I don't think that a model who is not comfortable with her own body and knows how to make it do a wide variety of movement can be "coached" into the approprate "poses" by someone else. He or she needs to be able to hear and understand what the photographer wants and to be able to deliver it without being told specifically what to do. Some models get it within a few practice shoots and are great ... and some never get it.