anyway, on topic: most people have spouted the same things over and over again and I want to say something original.
picture the scenario. a proud trainer of two years finally has a wonderful level 100 charizard. it may not be ev trained or have any particular Iv set, but it was mine, raised from level 5. alongside it, a level 98 blastoise, again raised from level 5 and this one nicknamed river, to add a personal touch, the charizard having been traded from my other game, unnamed.
I traded the pair to my brand newly-post elite four emerald version. Little did I know: a glitched version (eBay). one day, I turn it on to find that the game is no longer playable. not only had I lost the hours training swellow and the like, but my precious charizard and blastoise, my first two Pokemon ever. it was the only time I have ever cried at a video game and to this day, I name my party's water pokemon river and refuse to name any charmander evolution.