OPs tend to say alot of things...

Assassin's Creed is stealth. The very fist game was called "Assassin's Creed" and it was very focused on stealth. It became more action orientated later. ACII evolved but still retained stealth elements. They all have stealth but have been coming more action adventure ordinated.

I would agree with the Adventure/Stealth hybrid comment.

However one could argue the same with Metal Gear Solid these days. Stealth certainly makes the game easier but you can play the whole game without ever using stealth, you just might get fucked a few times. In Assassin's Creed there are plenty of missions where you cannot be detected. The Batman Arkham games have some heavy stealth elements that the player will want to use. Heck I actually use stealth more in that game than the other two :P