You could ask the same question about narrow focus about disciple of Dispater. Why is it a warrior class? Dispater, if taken purely as himself, seems more of a spellcaster, the sort to set up contingencies and wait things out. Then again, if we purely look at archfiends in general, you could make the argument for any class and have it stick. Most are better spellcasters, better warriors and better skill monkeys than any mortal.
For a note, I'm going to go with what Drolyt says about power and game balance, as those are aims of homebrewing, creating playable classes with balanced mechanics. On the other hand, your advice and ideas so far have been invaluable to me, and I definitely will be modifying the class as to some of your suggestions once I've time. And your suggestions are the other aim of homebrewing, creating interesting things that enrich the campaign world. To counter your "'more is better' is bad design" argument, I subscribe to the "dead levels are BORING" argument, which is to say even giving just a small flavourful ability at each level makes a class more interesting. It's not about power, it's about a balance of fluff and crunch.
On a side note, have you checked out Disciple of Glasya or Disciple of Levistus?