From what I've read in reports, there's a little bit of both. Apparently, when the ball is new, the grip is much better, thereby making it easier to palm/carry the ball. At this stage, they say the ball kinds 'sticks' to the hand esp. when shooting mid-range. As the game progresses, the ball gets wet and tends to slip out of the players' hands for the reason nowagimp stated.
I have a question now : so previous balls absorbed the sweat , thereby staying relatively dry on the surface....but didn't the increased weight of the ball from all the absorption affect the shooting in the latter parts of games?
This is a good question, whether adding several grams of weight changes the shooting effort. As players fatigue, or start to lose their legs, the ball also feels heavier. It also requires more strength to shoot an elevated jumpshot(shooting at the top of the jump), than shooting on the way up(momentum reinforces the shot), or a set shot where the feet or planted. I suspect that the human mind deals with this in a compensatory fashion, but dealing with extremes in grip is much more difficult.
As far a grip on the ball, I always used the seams of the ball on at least 2 fingertips to launch a jumpshot. As I picked the ball up for a shot, I would rotate it in my hand, slide it to feel for seams. This can also be done(feel for the seams) off the dribble for a quick shot, or after picking the ball up for a more deliberate shot, but the ball must slide in the hands some. I would quickly locate my fingertips at seams and use the grip of the seams to add the touch on the shot, get the proper ball rotation. With a grippy ball, sliding your hand over it to feel for the seams is more difficult to do quickly and then take a shot quickly, I just tended to get the ball stuck in my hand trying to search for the seams, delaying the quick shot motion. With a slippery ball, it is just a mess handling the ball alltogether. I always had more problems with new outdoor balls as I couldnt slide my fingers over the ball(a reflex motion) to feel for the seams. If I were a player, I wouldnt want to change my learned shooting reflexes because some marketing guy had a "great idea". Im on the side of the players on this one.