I usually never restart, going back to when I played the game hardcore we have to look at Red/Yellow/Gold.
Back then I had 3 copies of yellow (2 were borrowed from friends who just never asked for them back), 1 red and when it was released I had 1 gold. I would restart the Red + 2 copies of yellow but never my original yellow (save game is still alive, don't know how long it will last though). As for gold... I still had my original save file this year, but when I turned it on today I found that the battery had run out and my save was gone, disapointing as I had put in hundreds of hours catching the 3 legendary dogs and training my team etc.
So I just started my new game on Pokemon Gold today, I replaced the battery inside the cartidge... Almost makes me wish I had got Silver, so that I could transfer the pokemon between each every couple of years, replace the battery and start new, that way I could be almost sure to never lose them.
Moving on to Black 2, I still havent finished my original playthrough, don't own any other generation 5 games so will not be able to restart my game without losing my pokemon, I would like to though playing through with a dream team was one of the biggest reasons to restart. Playing Red with Charmander, Bulbasaur, Squirtle and Gyarados/Abra ready was definitely I enjoyed doing. Seems I have no way of doing it with current generations, as i'd need another ds and 5th gen game.