The main issue here, is a poor feedback as to why training isn't goin well, or why it is going well. There's personality conflicts. The Dorfs needs the right personality traits to be a good teacher, and have a teaching skill + the weapon/armor skill or activity skill. Then you also need the Student dorf to have the right mix of personality traits and the student skill.
It does work, but we dont have good feed back to make it work.
Argh, training via the traditional method is more complicated than I thought, and so easy to render ineffective. It wouldn't be half as bad, even without the feedback, if student skill was easier to come by, or if acquiring student and teacher skill were essentially minor speedbumps to simulate the slowness of starting an army from scratch before you quickly built up effective drill sergeants. The effectiveness of teaching is nullified by the fact that the most effective teachers can't drill multiple dwarves at once.
Although, the current system that makes it quite hard to train up melee dwarves DOES encourage the tactic of mass green civilians with crossbows deployed against enemy sieges. Isn't mass conscripts with effective and deadly ranged weapons that don't require much training a common real life tactic? ;P