Trading is definitely a good thing. I find that when placing settlements at the beginning it's crazy to try to get exposure to all 5 elements, but if you get really strong exposure to 3 of them you can trade for what you lack. Then you don't have to spread yourself too thin. Plus you know what people will do with what you trade them ... don't give up wood if you don't want people building roads.

Trading is good to increase own resources (I almost never trade 1:1, mostly 1:2, depending on how badly the other person needs my resources) or doing the inverse: getting down to 7 resources during another player's turn to prevent having to dump half of it if a 7 is rolled.

2 people who trade with each other will usually beat 2 players who don't trade.