In large part because the issue
isn't that you are "getting 4 back."
As I tried to point out, trade is an example of using Treasury income to increase monthly investment income. If you don't increase that income as much as you can with your resources, you don't do as well in the game. Indeed, from a mathematical standpoint (games theory, if you will), the whole GAME boils down to this important consideration.
As with all other expenditures out of the Treasury, you have to decide if the expenditure offers a reward in technology investment greater than another available expenditure. In general, it is good to focus on expenditures that result in long-lasting incremental contributions to monthly technology investment. Thus, getting 4d a year in increased income may not seem like much, but you have to start somewhere, and it does add up.
Would I recommend spending 8d a merchant to emplace them in Centers of Trade where they are competed out within a year or less, forcing you to spend that 8d repeatedly? No. But you can't just say "it's only 4d back so it's not worth it."