Some folks want Luffy to ditch Kizaru, for some unknown reason, and fight Saturn instead. Hello? Saturn already deals with the strawhats, if Luffy joins too, he's gonna get his shit rocket... and someone needs to deal with Kizaru.
Now that we know that Kizaru easily no diffed Sanji, it wouldn't make much of a difference if all strawhats jump Kizaru. Zoro still deals with Lucci, so Luffy is the only option.