A lot of people raised them for things like Battle Facilities (especially Garchomp, who evolves the fastest for some reason).
And, again, these are monsters available in the post-game in this case (Except Zweilous). Most of the middle stage evolutions available will be very near their evolving point, so why not have the fully evolved forms available on the route as rare encounters (Dragonite, fyi, is a 1% encounter when you fish in surfing spots)
I hated how the Unova Dex once again didn't even have Pikachu in it! Raichu is my favorite Pokemon, and I hated having to deal with most of the game with an Ampharos. I'm glad there were some trainers that used Pikachu and Raichu though, or I wouldn't get it ever.
My Black 2 Team:
Looking for: Any (I don't care if it was hacked or not) Level 1 Offering legit level 1 Oshawott or Pichu