Taking center pawn with adjacent pawn?

Something I've been wondering is if it's a good move to take a "free" center pawn with your adjacent pawn, or if it's a bad move - hence why you often see "free" pawns being left alone

While it seems the top move here is to actually take, plenty of master games show they don't take. This is a bad example, but often you'll see positions similar.

Is it bad for black to take here? Why? Because it's weakening kindside defense? I just wonder why i dont see more people taking towards the center, even though it doubles up the pawns doesnt it give more center control and win a free pawn? or am i confused

I mean, there's tons of different positions too not just my examples (which are pretty bad tbh). Overall, should you take towards the center with the adjacent pawn and double up your pawns, does this equal center control? Or is it a mistake
 

honestly I wouldn't take it generally, the pawn can easily be won back and even if you do defend it it ruins your structure, maybe just develop a piece instead? Idk

Not at all bad. If white takes with queen you have a tempo with nf3. If they play c3, Smith morra, take that pawn also and then just play e6 and develop knights

the entire point of playing the Sicilian is to not allow two central pawns in the center, so if white gives you the chance to add that imbalance, you take it