Lets see, new campaign, new multiplayer maps, buffed up engine. Beats me why I would buy this when I still have Golden Eye lying around here. Really it's just slight upgrades over that, and they probably won't even have cheat mode in it (LAME). I mean who needs any of these new fangled new things when I have old fangled old things?
You're question really makes no sense. I will buy MW2 because I want a new Modern Warfare game. I will buy Super Mario Galaxy 2 because I want a new Super Mario Galaxy Game. Will they be better than the originals? Who knows, they aren't out yet. But I still want more of what I enjoyed in the first games, so I will buy the second games. Seems like a pretty silly question.
Also I do not think single player games are rental only. Most of the games I own are single player games, or have multiplayer components I don't care much about. Maybe if you don't have the money a person might desire to focus on only purchasing long lasting multiplayer experiences. But that's not me. I buy games I want.
As a value proposition you personally may not find this game warranting your 60 dollars, because you are content with the product you have. However there will be millions who are willing to pay for the new maps, new campaign, beefed up engine, ect ect. If you don't want it, that's your prerogative. However I think it warrants the cash just as much as any sequel to a beloved game does.