I was taught to ALWAYS take the d4 pawn in the Sicilian. NO MATTER WHAT. Well, that is how they taught things when I was in high school. Anyway, I can tell you from a friend of a friend let's say, that, well, your center tends to just collapse if you don't take d4. I mean this was drilled in to the point of just being part of the opening, as in, "oh I just learned this great chess opening, it's called c5 takes d4, pretty cool, huh?" Also, the Sicilian defense, in my opinion, is bit of a misnomer. It is really more of a counter-attack, and taking on d4 is attacking, albeit usually in a somewhat limited sense of the word. If you're playing white and move to d4 and it isn't captured, seriously consider advancing the pawn to d5. It basically says to black, "we're playing my game now, joker." And your knight is in my way." That's fine, but if you are the type of player who wants white to just play their game, then don't play the Sicilian, play the Hippo or something. Obviously these are generalizations. Anyone who can point to a Sicilian where it's not advised to take d4 as black, I would be very appreciative. No really, my entire adolesence and young adulthood would surely be given new meaning.
Interesting.
Nice and useful !