A. You can catch some pokemon from Gold + Silver [Ledyba and Spinerak etc.]
B. Updated Gyms, + Elite 4
C. Battle Frontier.. =]
D. Magma and Team Aqua both in game
E. More of a challenge, and more hours of gameplay, especially w/ BF.
If you have the extra money, and have nothing else to do, I suggest buying it
But thats just me.