The GTS is amazing in Pokemon X and Y so in reality, you don't need to buy a gen. 5 game(or any others for that matter ). Unless of course, you don't own a gen 5 game then I definitely would buy it for the new Pokemon experience that you've never had before.
While Black 2/White 2 rely a bit on storylines from the previous games, I wouldn't say you need to play those before trying the second ones. If you're using it primarily as a transfer device though, it might just be cheaper to get the originals.
With that said though, Pokemon General isn't the place to be asking questions solely about the video games. If you're looking for more information on any of the fifth gen games (including more opinions), try posting in our Fifth Generation Gaming forum.