If it ain't broke, don't fix it. Certainly, the blokes at Ubisoft follow this rule - Assassin's Creed: Revelations is pretty much a reskin of the previous games, with a couple of bells and whistles cobbled onto the side (bombs and hooky-things are a nice addition, kudos) but the blood and guts look the same. And rightly so, it ain't broke.
However, if something isn't broken - you certainly don't need to take something which IS broken and mash it to the side; hoping that it will grow some sort of symbiotic relationship and miraculously not be crap. Sandbox games are brilliant because you are allowed to roam wherever you like. This is especially true with the 'screed series - if you want to climb that impossibly high tower and then throw yourself off the top, then go do-it to-it good buddy! However, what you DON'T need is the constant reminder to "Quick, stop whatever fun thing you're doing and come play our arduous tower-defense game which is super-duper clunky on the PS3 controller and takes fricken ages!"
It's like the whiney girlfriends in GTA: San Andreas all over again.