Yahtzee strikes me as a gamer who really likes to inject his own flavour into the storyline of a game where the supplied storyline is lacking or deliberately "spacious", and I indulge in this as well.
While I think a lot of people are finding SR2's digression from the path to gritty realism a welcome change, I'm going to put myself up as the devil's advocate here.
I would say that I (and Yahtzee) have vested interest in spicing up games with my own storylines, motivations, etc., but I find that in more realistic games this is generally easier to enjoy. I say 'enjoy' and not 'do' because in more realistic games you tend to get more immersed in everyday expectations - you walk around Liberty City and feel like "this could be real", and then you go and pick up your date for the night in a semitrailer caked in blood and legs. That is definitely more jarring (and hence entertaining) than doing the same in a colourful whacky version of reality, e.g. SR2, GTA1.
In fact I would catch myself trying to offset the absurd comedy I injected into the storyline of my Niko character by investing time in the more realistic aspects of Grand Theft Normal Boring Life. The TV ads and stuff? They fit the bill perfectly for tipping the scales neatly back in between gritty and hilarious, immersive and entertaining.
I've seen the gameplay movies of SR2 and it doesn't really strike me as a game that concentrates much on realism. To me it's like they've taken the idea of "user-created storyline" and incorporated it as part of the game - encouraging it by making the world less realistic and putting more effort into the whacky shit like "doing x to old granny y". To me it's inbalanced, and would get boring after a while.
Not that GTA4 didn't suffer from repetitiveness of missions being variations on "drive here, kill him, protect her, steal that, deliver this, yada yada yada", but I still prefer it over more stylised sandbox games like SR2.
Just as a sidenote, I think the whole balance thing between realism and user-created whackiness is enhanced many times over when you're with like minded people. Classic example: BF1942. When that game first game out and half the server population was learning to pwn and the other half learning to blow as many teammates sky high as possible, it was bliss.
Anyway, that's my two cents. Thanks for hitting off this chain of thoughts in my brain, Yahtzee.