...but with the way things are now I would rather have seen the Assassin as a Rogue build than a whole new class.
This would make a lot of sense if it was planned from the beginning to be this way... but you know what I believe happened?
The Assassin was a repurposed Ninja when the Ki power source got nixed.
I suspect that way back when they were putting Player's Handbook 3 together, they had a partially fleshed out Ki power source. After all... the removal of the Ki source was fairly far along in the design process... and this is evident from so many aspects of the design that remained with the Monk. The Ki Focus, the Full Disciplines etc.
What happened to all the other class stuff they designed for Ki? They wouldn't just throw it out whole-cloth... they probably took what they had and tried to find ways to re-use them. And looking at the non-psionic classes that were in PH3... the choices they went with and the power sources they were assigned to seem awfully arbitrary. The Runepriest has
nothing in common with the other Divine classes, especially considering it doesn't use Channel Divinity and doesn't actually get it's powers from the gods. So what if the Runepriest was actually repurposed from another class? The Seeker? I've heard a number of people talk about how the "magical archer" archetype is actually fairly prominent in anime (granted I know nothing about it, so I cannot confirm whether or not this is true), but using the magical force of Ki to make a controller archer seems extremely plausible. And the Assassin? Heck, this was the easiest thing of them all. When the announcement of the Assassin playtest happened practically on top of the announcement that the Ki source was being jettisoned... I immediately thought to myself "Well... that's the Ninja class being given to us right there."
If my beliefs are in any way true, then we really can't blame or hold WotC accountable for the decision to make the Assassin its own class, but rather should look at it as an extra "perk" we got for being DDI subscribers. After all, they could have just thrown out all the hard work they did on the class all together... but they instead decided to give us a little something extra as a reward.