Re: Gotta catch em' all !: Club
Trading will cost points, but just the cost of the pokemon.
Legends will count as part of the Pokedex. Honestly you bring up some good points. The side quests missions are meant to help you fill your pokedex. There will be other pokemon on those mission though, the rare ones that only show up once in the gcea red and blue.
If lengeds were usable it would get a little out of hand. To some capacity Legends could be used like a get out jail free card. Maybe that should be the reward for going for them, having the legend on your team for one battle or mission.