The answer depends more on what type of game you run and what the party's goals are than on anything inherent to a given occupation.
Among the things Mercantylers tend to be good at: Rhetoric and Intrigue. Consider the uses of these skills. Does the party tend to get involved in business or political wrangling? Do they often need to persuade someone to do something or adopt a certain viewpoint? Do they need to gather clues in a social setting? More directly, are they in need of someone who can handle business negotiations, contracts, and the like? If so, then there are clearly advantages to knowing a mercantyler. Mercantylers are literate and often know one or more additional languages, which can be a plus in many situations. Additionally, mercantylers may have valuable contacts at various social levels that other characters may lack.
In one campaign I ran, one of the PCs was a mercantyler who functioned as the group's logistics expert and negotiator. He made sure they had all the necessary equipment and provisions for their journeys, and was very useful at gathering information about the local situation when they arrived in a new settlement.