Well, all that invisibility is going to be handy when you've got controller HP.
Yeah, that seems to be the major reason. Which is an alright reason.
But they have a lot of abilities that don't actually make you invisible, just give you combat advantage. These are the ones I'm wondering about.
I'm actually a little concerned that the class doesn't appear to output that much damage either. Especially the Con based build. Your extra damage is smaller than Rogues. You do get to use bigger weapons with your attacks...but at the same time there's a lot of role playing reasons to use small weapons(you can conceal them for one...kind of useful as an assassin).
Plus, a lot of their utilities are very useful in a purely role playing situation...which is kind of cool. But at the same time, they are pretty much ONLY useful in a solo situation where the assassin is attempting to sneak into a building by himself, take out the guards along the way and kill someone then sneak out. Which is good, because that's what an assassin should be able to do. But in an actual D&D game, I see this situation happening almost never. No one wants to sit back and watch the assassin go on a solo mission. Plus, the mechanics of 4e are designed around group battles. It creates an odd situation.
Take, for example, the feat that makes it so that enemies who are unaware of you don't realize that they've been shrouded. It's a cool roleplaying feat. You can sneak up on a guard, they don't know you are there, you shroud one of them 4 times. Then you jump out of cover and hit them with your full shroud bonus. But the average 4e party is pretty much incapable of sneaking up on anyone. Even when they are capable of sneaking, they often choose not to due to time constraints or pure overconfidence. So, unless the assassin is by himself, the feat will almost never get used.
I suppose it COULD be used in combat, if you were invisible and decided to wait 4 rounds without attacking while you shrouded an enemy. But that seems a rather poor way of helping your party in combat.