I had a copy of Pearl, which is what I played, and a friend's copy of Diamond which they did not want anymore. I replayed Diamond about 3 times over to get the Master Ball so I could easily catch any legendary pokemon in my Pearl, eg. Giratina.
Looking back this was pretty pointless because the legendaries are not actually that hard to catch - you just need a lot of Ultra Balls, and a lot of patience.
I could never actually replay the games once I had completed them though - I have about 400 or 500 + hours of gameplay on Pearl and White 2, with a complete pokedex on White 2 so restarting the game would be like sticking a knife into my eye - painful and traumatic.