Everyone has their likes and dislikes. For example, I dislike Raichu. A lot. But I don't go around bringing that up at every opportunity, harass people who do like this pokémon, and I don't especially put a lot of energy into hating on it. Raichu have appeared in a couple fanfics of mine, and you couldn't even tell by reading these inoffensive (and in one case, positive) portrayals that they're my least favorite pokémon.
To sum it up, it's not that people dislike certain pokemon, characters, generations, and so on, but what they do with that dislike. I couldn't care less about the opinions of people who don't like Pokémon at all - unless they start harassing me because I do like it (or on the reverse, fans harassing those who don't like it just because they don't like it - it's a two way street). I'm well aware of the fact that my own favorite Pokémon (Plusle) and favorite fanfic genre (Pokémorph) get a lot of hate within the fandom, and while I might defend my own love against specific insults ("lol pokemorph fic writers are fur***s" being a particular one that really gets my goat), mere disagreement without any personal attacks contained shouldn't be bothering people as much as it does.
I don't "get anything" out of disliking Raichu - but seriously, people, if you like something I don't, realize that my dislike isn't a personal insult against you. >.> (Shipping wars are a good example of this sort of thing). If you hate something I love, on the contrary, my liking the hated thing shouldn't be treated as an odious affront to your senses.