Alot could be done with this.   Random spells, random really powerful attacks at the end of trees, etc.  
To me this would add some fun into getting new heroes.  As soon as you got one, you'd be checking out their skill tree's to see what potential they have, though I think it would be even cooler if you could mask it so you couldn't see what it was going to be until you got there.   So there could be "mage slot 5" that has a random skill, and you could define 4 or 5 skills to that slot.  Each mage would only get one.   Even if we added the old UI so they could pick from a choice of a couple abilities when they reached it, but it would be a couple randoms.