Seriously, how should it work to split them all up? You get into total summer and heat into hardest winter from one day after the other, because you didn't buy autumn pack? Only have snow and sun now but no rain?
No, seasons have to be in a complete pack. Anything other would be nonsense. It's okay to split up creatures as I want to have vampires (and fairies) but no zombies or werewolfs (never played them on TS3). Maybe even pets if they focus better on the stuff. I could think about splitting holiday packs (what would be ok for me, if they have more details then). But seasons? No. really. If they want to give us good weather changes, everyone needs all seasons and whether in their game.
