The cool mentor character always dies. Maybe that's why they're cooler. I rather think they have to die because they are so cool they make the hero look like a stupid pansy. ;)

You're just not interested in Dantrag, but let's be realistical. Underdark or not, Drizzt was still rather young at that point. there is no canon information if Dantrag was better than Zaknafein or not (except Drizzt's bitching, but I don't count that), but even if he was less capable than Zak, Dantrag was still several levels higher than Drizzt. And he must have had a bunch of battle experience with other creatures as well - you don't get to be (second-)best fighter of Menzo just by fighting drow. And deep down, RAS knew that there was no reason why Drizzt should defeat Dantrag. So he came up with that stupid speed-bracers thing. C'mon? Dantrag is several centuries old and he's had those bracers for quite a while. He is arrogant, yes, but not too arrogant not to notice that those bracers have disadvantages as well. If he had problems controlling his speed (and, as a roleplayer, I'm a bit bitchy about this, because according to the rules, he shouldn't have any problems), he would have noticed them much earlier and found a way to deal with them. He wouldn't have been surprised, suddenly, during that fight with Drizzt, that those bracers also have disadvantages. No way. Sure, Drizzt was a more challenging opponent than most people Dantrag usually sparred with, but whatever RAS wants us to believe, Drizzt is not so exceptional that there are no other fighters his level around, fighters with whom Dantrag no doubt sparred at the Academy. It's just so absurd. Such a cliché, "the hero wins the impossible battle by turning the villain's advantage into a disadvantage", also known as "My Sue is doomed, I'm going to screw the rules as well as common sense to make him survive and save the day anyway!"

Sorry for the rambling. ;)