This is exactly what I was getting at:
I am coming at this from a fellowship perspective. When I recruit I look for players with boosts that will compliment the existing trade balance in the FS. My experience has been that players with multiple traveling merchants of each tier are less likely to view the trader as the primary way they should be obtaining their non-boosted goods, and as a result, do not trade as frequently or at as high a volume. It also most likely means they are not producing as much of their boosted goods as they could. This is because the space they could be using for more or bigger factories is being taken up by merchants. The math laid out by
@crackie shows just how inefficient this is. The end result is that there are less of that players boosted goods flowing through the trader to the rest of the fellowship. That, in turn, shifts the burden to other players with those boosts to increase their productions to cover the short fall created by the player who isn't trading, otherwise a trading imbalance develops.
These players also tend to get stuck as they get farther into the game and the demands on their goods increase but their production isn't able to keep pace.
To be clear, I'm talking about players with multiple traveling merchants (I've seen as many as 3 of each tier, so 9 total in some cities), not players who are using a couple to get past a tight spot in their game.