Portuguese colonists
I play as Portugal but at very easy level though.
I find trading posts are good in the ivory provinces in Africa which are not worth colonising, they add to COT values in Tago. Also if you have enough colonies and TP's far from Europe there will be little competition although it is expensive to place your merchants.
TP's have two other nasty uses. Put a TP in a province full of hostile natives next to the territory of someone you are at war with, the AI will then invade to destroy your tade post which means they get to fight the natives, waste time on a useless excursion and suffer attrition. Good value for 5 ducats and worth the risk of natives destroying the TP.
Another nasty use is to place a TP to attract pirates near someone elses colonies.