I've never been unable to beat the game with my party combinations, so picking up The Dagger of the Star +5 now and again isn't inconceivable for me. For one thing, I find it's a stylish weapon. On my most recent playthrough I gave it to Mazzy to dual wield with an endgame short sword. It's handy for occasionally allowing her to blink out, persuading a large group of melee enemies to keep their attention on the tank. If nothing else it spares you having to pick up Hardiness or giving her your more protective gear.
A Thief could surely make use of it for added backstab opportunities in combat, although I imagine it would just be a bonus method for them. Creating a build/playstyle around this weapon is unrealistic given how late you get it and the fact that it relies on probability. If you've got a Thief-Mage that plays off of contingencies and invisibility spells to achieve mid-combat backstabs, though, then The Dagger of the Star is a more thematically applicable choice then any other late game Roguish weapon (quarterstaves being not at all Roguish, in my opinion).
I wouldn't try to morally redeem Sarevok or Irenicus using the reasoning that you have provided, personally. I think their morality lies in the fact that they have a goal and a purpose. They have morals (no being can be a God without possessing some kind of idealogy, after all), they just aren't morals that we consider to be acceptable. But both villains fight from a place of belief and aspiration. If they are better than the Luremaster it is because - as you say - the Luremaster seems to have no purpose except to arbitrarily cause grief.
That said, I do agree with @ineth that the likelihood is that the Luremaster has been transmuted into a difference consciousness than a human one. Much as a man afflicted with lycanthropy might harm his loved ones during the full moon, there is a switch in psyche that can't be accounted for by presuming to impose the mindset of a rational, sentient human upon the villain.
I try to mix it up, but in the long-term I'm guessing that Arcane classes take priority. I have done plenty of Sorcerers, specialist Mages, generalist Mages dualed to and from the Thief class, as well as a multi-class Thief/Mage, and a Fighter dualed to a Mage once as well.
I have played several melee characters as well, although not of every class. Still, given the number of playthroughs I have done, the most noticeable omission is probably Cleric. I know I once completed the game with a Cleric/Ranger, but that was well over 10 years ago and I don't remember playing one since..!
I'm just enjoying the layout of the NPCs, personally. Jaheira & Khalid together checks out. Edwin & Minsc? Hilarious. Xan, Dynaheir and Neera are likely having a forensic dialogue to try and understand Neera's stoopid powers (yeah, I said it). Viconia & Branwen are probably being strong, divinely-empowered women in the corner, commending each other's successful independence, with Viconia being perhaps a little derogatory towards the symbolism implied by Branwen's servitude under a male deity. The fact that all the shorties are in one corner (with the exception of Montaron) is something I'm going to call you out for re: racism, just for fun. And then there's Kivan and Ajantis rivalling one another for Safana's false affections. A fallen Ranger or fallen Paladin, I wonder which one it shall be?
Dynaheir - I am having difficulty putting this one into words. I enjoy that her brand of lawful goodness is of a somewhat more academic, unfeeling variety than I recall seeing anywhere else in the games based off of D&D. She isn't a Divine character and never references a Divine nor bleeding heart motivation for her ideologies. She seems like a woman with great potential who has thought a lot about the kind of world she wants to live in and is taking practical steps to try and create that kind of world. Style points for that, and further style points for her portrait, voice, and Mage specialisation - she's got no time for empathy. If there are forces in this world that are going to undermine her philanthropic ideals, she's gonna fireball them out of existence and get on with the job.
Xzar - It has been an age since I played with Xzar, but I really must do so soon. He is no fool - look at his wisdom stat! The insanity in his design yet seems to have some kind of rhythm to it that you don't see in many other Mage characters (Dynaheir is the only other high wisdom mage in Baldur's Gate, if you don't include Aerie, whose personality and narrative don't hugely incorporate the mage class). Consider that he switches between seeming madness and a very calculating mindset and I suspect Xzar is far more in control than out of it. Xzar is an interesting guy, and as @Stupid_Puns has said, his early introduction really supports how memorable his personality is.
Alora - I want to feel a little bad about this one as there's no reason I shouldn't find her very annoying, but I just don't! Cheerful; great portrait; excellent performance from what I recall of the 2 or 3 runs I have done with her over the years. She makes a pleasant alternative to Imoen or Safana, to whom I often find myself resigned for much of the game when I want a well-rounded Thief from the early days of adventuring. There's also the shortie points. If you're getting a group of 6 together, you really should want a shortie. I think she's the pick of the litter in that category!
Jaheira - More and more lately I'm finding the strength to kick Jaheira out of my BG2 parties, but in my long history of the BG saga, Jaheira has surely had the most presence across all playthroughs in the many parties I've mixed together. Jaheira's great. In BG1 She is consistently useful from level 1-8, faithfully filling a melee role in my parties as a defender or, more occasionally, a two-handed weapon master. If I know I'm going to be using her when I import into BG2, there's a 100% chance she'll be coming with me through BG1. Furthermore - with the exception of Viconia - there are no Clerics/Druids available in BG1 that I enjoy very much anyway, and as I've not played a Cleric in over a decade or more, Jaheira's mechanical value continues to soar. In terms of narrative? Jaheira's got the highest cards in the deck among all the NPCs once BG2 kicks off, and her personality is excellent if you ask me!