Well, I have a funny story on this topic:
It was my first play-through of Sapphire, and I had a majorly over-leved Swampert before the third gym. I had no other pokemon except a Maril I caught due to it being one of my favs. It was lvl. 19 or so, and my Swampert was 39. I showed my game to my cousin, who played around a bit, and gave the game back to me. What I found was horrible: He had released my Swampert! All I had was the Maril, and the gym was electric type, so I was screwed. The worst part was he had saved. I began my impossible quest: Make this little Maril a force to be heavily reckoned with. I grinded that Maril like there was no tommorow, and ended up solo'ing Wattson with my new Azumaril. I continued through the game, catching a couple more pokemon I rarly used, until I came back to Wattson's town (I forget the name, it's been awhile, :0) and that was before 6th gym. I then found out my Swampert had been put in the day-care. I raged, and raged, and raged some more, but in the end it turned out great, because my Swampert/Azumaril over-levled combo was enough to set me up for the rest of the game. In all my other games, I stive to make all my pokemon equally leveled. THE END.