As I read Gordon talking about MMOs possibly harming single-player games I was struck by the sad question: what are we paying for?
There are many ways to measure the value of a game, many comparisons to use. All of them are tangentially based on dollars, which are money, so none of your Euro nonsense here.
Time played per dollar
Fun per dollar (I believe that fun is theoretically quantifiable, but it comes in too many forms in too many ways and is too subjective to be easily figured out)
Fun per time - concentration of fun
Concentration of fun per dollar
Note that I am leaving out the social factors for now.
In my experience single-player games have a much higher fun per time than MMOs. Not needing to share a world with others allows for greater depth of mechanics, more gameplay options, and much less attention to nitpicking balance and risk/reward. For example, I like that in a single-player RPG I can sometimes go to a NPC and just buy something nice, with plain old gold. No rep grind, no tokens, no bosses, just here's some gold and wow how does this person have that for sale? This would probably be an awful idea in a MMO since inevitably someone would miss out on it or someone would monopolize the rare spawn sale or whatever. Oh and difficulty sliders, those things are awesome.
MMOs tend to compensate by having lots and lots of time, time which can boost back up the fun per dollar. In this way they're a bit like bad candy, but it comes it two pound bags! WOOO! Perfect for spreading gum disease and diabetes to children while pretending to be nice. This leads to a very low concentration of fun per dollar. It also means that if you can't play a lot, you're going to have some value issues.
Does the social factor make up for this? That's the key element, the one major advantage of MMOs. The social interaction or at least epeen contests must be enough to compensate for the lower fun per time per dollar. Back when I sometimes played Goldeneye with my brothers, fighting three bots wouldn't have been much fun compared to fighting two brothers and a cousin. The actual play wasn't particularly amazing, but the social aspect made up for it. It added another layer. That layer being a lot of yelling.