Ok I was wondering why would you want to unlearn a skill??? Are there so many lines of skills that you may not have room for one later in the game that you might need at a certain time?
No, it's because you'll find that you won't have enough skillpoints to go around, and sometimes you'll have to reorganize your skills.
For instance in Act2 you'll meet different kind of elementals (fire, water, air and earth) in different areas. Once I killed one, I can check my trophies for their weaknesses. Then all I have to do is unlearn my previous elemental spell, and learn the one that my enemies are weak to in that area, which makes it a whole lot easier for me.
So to answer your question, it's purely strategical. Plus, if you find out you don't really need one of the skills you learned, you can always undo it this way.
Tovi 