Several reasons. Theres no guarantee one can log in at anytime, this is why people get up in arms when companies like Ubisoft mandate a persistent connection to the servers to play their games. What if my ISP has an issue? Or the Nexus login servers? Right now, the Client crashes anytime I try to log in, while OBMM and FOMM work fine. Futhermore, I get more of my mods from other sources than I do from the nexus, (and I suppose it would seem silly to use something called the Nexus Mod Manager to manage those as well, but Id hoped the successor to the prior managers would retain that functionality) I don't see a need to login to manage those mods.