In any case, the present set up re: background and theme suggests that classes are customizable to at least a limited degree. Fine. But, and I am not the first person to point this out, if assassin (or bard, or paladin, or whatever) is reduced to a theme to whatever actual class, then for any given character of that type it requires both sacrificing a customizable slot and also leads to a bunch of that character type being mechanically identical. IOW, you lose customizability because you've opted to play something that isn't a base class. I think that stinks.
It seems to me that a lot of folk advocating that class x should be a theme at heart are motivated to say so because they don't like that class. That's not a valid reason for a design decision, and trying to dress it up as something otherwise is, at least, disingenuous.
Which is not to say that some objection isn't valid - I want a separate assassin class but I'll acknowledge that the generic and bland rogue covers the same abilities and so some mechanical differentiation would be necessary. I make the same case with the cleric. (I was really sold on the notion of separate priest and cleric classes, with the value of cleric largely equaling paladin, as they originate from the same sources.) The real question is how to make such a mechanical distinction and make it work.