the palm-top tiger
Well, trade evolutions offer a bunch of things. You get to fill another spot in the Pokedex, and you get a more stronger Pokemon. A good example of this is Gengar. Haunter is rather weak compared to Gengar. But, as with many positives, there are negatives to balance it out.
Trades can be tedious if you don't have anyone to trade with. You could be spending days looking for someone to trade with, whether it be at home or on the internet...and usually, people on the net would want something in return. Also, some trades could require the use of items, such as the Reaper Cloth for Dusclops to evolve into Dusknoir, or the Metal Coat for Steelix and Scizor. It could take a bit of time to find those items if you don't exactly know where to get them.