They are, but they add a lot to it. Especially Brotherhood. I was surprised with how much Brotherhood added to the franchise. And it has some very good story elements that cannot be missed. Revelations adds even more, but its running on such old framework that you can tell its time they changed things up. It has some pretty frustrating missions, but at I think the story so far more than makes up for it.
I guess it really comes down to how much you like the character of Ezio. He's one of my favorite characters of this generation, so I felt compelled to stick by him and see his story end.