Trading was part of the original game's marketing. In theory they would sell more copies as you would either buy an additional for yourself, or make sure a friend or family member bought one, or you bought one for them. Personally that's a little narcissistic, and I don't think a game should ever cater to or design around a personality disorder.
Pokemon GO though has not limited what you can or cannot catch to a specific version, nor have they made your team choice effect that in any way either.
Point here being, trading is NOT a design imperative for Pokemon GO. You can (without trading) catch every pokemon available.
I know that for me personally, beyond trading pokemon that were not in your respective versions, or trading because the mechanism of trading caused an evolve or something similar, I didn't CONTINUE to trade beyond what I HAD to. Because the trading mechanism in and of itself... was actually quite cumbersome. Additionally that trading mechanism opened doors (even in the original games) for significant ways to cheat.
(1) Really no need for it
(2) Another potential way for cheaters/spoofers to circumvent intended design