hand-waved assassination table
I freely admit this is coming from my primarily 2-3e experience, but I broke out in hives looking at that 1e DMG Assassination Table. If you're considering tinkering with the class to make it a functional part of your campaign world, consider further codifying the steps that could be taken to augment that Table!
In romanticized media, the assassin is portrayed as meticulous and calculating. He doesn't pull the trigger until he's as close to 100% certain as he can get. A 35 or 20% certainty is nowhere NEAR acceptable odds for this kind of character. I'm looking for a guy who, if he does the legwork (with the help of his team) has a high probability of ending a boss-fight in one shot. If he's doing this all the time, then he's breaking the game for everyone, but if you provide a couple of key, assassin's notes for each boss character, that makes it clear who's open season for assassination. Everything else is a sneak attack, and assassins function as thieves 2(?) levels lower (I think) for dealing with henchmen etc.
I think there's a lot of trepidation about party balance going on here, but every class has a roll to play. For the most part, an assassin is a junior thief in the party, assisting with reconnaissance and skirmishing. Every once in a while, he can turn an arduous battle into smooth operation. On the other side, I'm detecting a lot of DM horror for their precious boss encounters. But if the character is willing to imperil themselves doing the research and recon, what's wrong with an instant kill? If you want action and excitement, imagine the hornets' nest that will be stirred up in the wake of this important person's death!
Anyway, to sum up: I'm getting shrill about this because I've been trying to get some kind of solid sniper/assassination rules going for three or four campaigns now. I'm frustrated by nerfed rules that support cowardly DMing. I agree that no one wants to sit at the table with someone who's just one-shotting everything and hogging the spotlight. So make the rules of research and recon more concrete. If every assassination requires significant preparation and support, then players will choose their targets more judiciously and involve other players in their plans. Add some details to each major NPC in the narrative; weaknesses and precautions and how much each could bump an assassination attempt one way or the other. Throw in a random leverage, generator table for other NPC's that might cross the assassin's path. Detail the black-ops wings of any organizations employing assassins in the city; their relationship to their contractors, what they demand of them, and to what degree they support them. Pack this all into an assassin's quest adventure that clearly shows how this could all work without alignment restrictions or having to exclude other players at the table.
As for this sub-class argument. Sure. Technically, we could function with just Red, Yellow and Blue. All the other colours are just variations or combinations of the three primary
classes colours. But DAMN that's a lot of work and stretching the imagination and rationalizing to differentiate the six fighters in your party. Some purple, orange and green is helpful and gives the player and DM a basis to work off of. Does the game need Fuchsia? Probably not; but if a player is willing to give it an honest go and isn't just brewing up some monstrosity off the Min/Max boards, then sure, why not. I encourage anyone with profound misgivings regarding paladins to read 'The Mongoliad' or 'The Ill-Made Knight' both of which, I promise you won't be able to put down once you pick them up!! Both offer excellent insight into the lost western martial arts and the functional value of the code of chivalry.