Main problem with trading is that it has the potential to turn the game into an illegal-RMT fest. If trading came to PoGo? I'd get a card table and go down to the local pokemon park and start buying pokemon from the kiddies who don't appreciate their value and selling them to the 30-something hardcore players for a profit. It would be an absolute shitshow, because the moment you can trade things with other players, you can RMT. I think Niantic is keenly aware of the risk that trading would destroy the game like this, and has held off on trading for that reason.
Particularly since this game is missing a key element: A permanent sink for good pokemon. Before the gyms were sort of that - your best defenders might spend long periods of time in a gym and you'd need others. But now we don't even have that (and it's temporary anyway). So long time players end up with an excess of good pokemon - and if there was trading, they would trade that excess around, and people would get extensive collections with less gameplay, and get bored and quit.