When it comes to determining which concepts get classes, important considerations include:
1. Unique mechanics
2. Unique place in the game/world
I think assassin can and should be given a class based on those criteria.
Sure, it’s easy enough to give a rogue the right skills and call him an assassin, and it makes perfect sense. It’s just a job title.
However, similar logic would allow any class to be an assassin. Fighter, wizard, etc. You kill people for a living—you’re an assassin. Good enough.
But...it’s not. Although there was a lapse in 2e, in every other edition since 1e AD&D the idea of a unique class to represent the iconic assassin has been there.
1. In every instance unique mechanics were applied, many of which carried over. I’m only minimally familiar with the 4e assassin, so I won’t mention it much. But here are a few things that I see as iconic D&D assassin features.
a) Poison usage. They are better with poison than other classes.
b) Minor magical abilities. They study arcane magic to support their other abilities.
c) Rogue-like in combat, but better than a rogue (through an expanded weapon selection, better HD, whatever)
d) Special ability to perform better death/critical sorts of attacks.
That seems about as distinct from rogue as ranger is from fighter.
2. When we talk about assassins, we primarily mean the kinds of people who will be sent on the job by an assassin’s guild (and their corollaries who may work for any organization or individual). If you go to the local guild, this is what you expect to see. They are trained specifically in killing—more so than a rogue. They are better at it. They focus on techniques (such as poison) or death attacks that kill more easily than a rogue. They learn whatever they can—including dabbling in arcane magic at higher levels—to make them better at what they do.
The archetype has expanded beyond D&D into the fantasy genre in general. It’s found in books with no direct connection to D&D, MMOs, PC games, console games, etc. Few of these attempt to squeeze it together into a thief or rogue class or category—despite the fact that it could be done easily enough.
I get excited about playing an iconic assassin with unique mechanics, spellcasting at higher levels, and clearly distinct from rogue.
I’m not saying that assassin can’t be done “well enough” with rogue. What I’m saying is that it deserves to be treated with enough respect to be given its own class, just like rangers, paladins, and other traditional D&D classes.