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Not necessarily "harder", it would just make the game less challenging if you didn't have to trade for certain Pokemon, and could attain them in your own game for collector's purposes.
I for one, like the feeling of finally getting a Pokemon I've needed to complete my dex through a fair trade. Feels good.
That's the thing though: the level of "Challenge" is not based on the game itself, but who you know and/or how much money you have, and I feel those sorts of "Challenges" don't have any place in a well made game.
If you have loads of money to buy an extra handheld of the generation and the other version, or a friend who A: has a Pokemon game that s/he plays regularly, B: you trust to give your Pokemon back, AND C: you can regularly and easily interact with, it's just a slight inconvenience. It's only a "challenge" when money is sparse and your friends don't meet all three criteria, or don't even like Pokemon at all. That strikes me as bad game design.
Edit: That said, I do like some of the ideas proposed in this topic, to make it possible to evolve them without other players, just less convenient.
Frankly, I'd like to see more Pokemon multiplayer features go the way of Join Avenue and the Funfest missions: Useful features that can be made more convenient by interacting with other players, but not impossible without them.