Set items are not very good for the most part, nothing near like completing a set in Diablo.
Once you start searching for end game items, trade is most certainly going to be used a ton.
I think you're missing the point. That is just one faction reward, there are 10 total. The trade faction is hot garbage. Nobody will be using it because it will be easier to get end game gear with circle of fate than it will be in trade.
EDIT: Here is a taste of the difference in the two factions. The Circle of Fate rewards are too good, and prophecies are way too good. The tradeoff to just being in the Merchant guild is too big, you're better off self farming in CoF
Circle of Fortune
Rank 2 allows for the use of fewer Runes of Ascendance with a 45% chance to preserve the Rune when crafting.
Rank 5 provides a 35% chance to double Echo rewards. This includes Unique/Set Echo rewards.
Rank 6 gives Uniques twice the chance to have Legendary Potential.
Rank 9 drops the entire Set any time a single Set item drops.
The Circle of Fortune provides additional opportunities to target farm by allowing the play to spend Favor on Prophecies. Each Prophecy has a predetermined objective and reward, allowing the player to choose the reward they prefer before accepting a Prophecy. These rewards can be Unique/Set items.
Merchant's Guild
Rank 2 allows you to purchase Set items.
Rank 3 allows for the purchase of Uniques without Legendary Potential.
Rank 6 opens up the ability to buy Unique weapons with Legendary Potential.
Rank 8 allows you to purchase all Unique items, including those with Legendary Potential.
Rank 9 allows for the purchase of Legendary Weapons.
Rank 10 opens up all Legendary items for purchase.