I'm new here, so I don't know if this has been covered yet...
The - what I call - mercantile route is most useful in scenarios where you are under-manned (i.e. you don't have enough troops) or your troops are of insufficient technological dvelopment to attack your enemies (i.i you have cannons and they have marines defending). What you can do in such situationsis using a diplomat or spy, bribe the city to revolt. This is also useful if you are going onto another island and need a safe place to land your troops.
Also having some spare money i useful in case you need to rush buy a unit or wonder (to build it before someone else). I raise this money by not concentrating on science and building markets, banks etc. Also it is important to build lots of roads as they add money as well.
Science can then either be exchanged or stolen, although this means you can never get really ahead of the other civs.
The - what I call - mercantile route is most useful in scenarios where you are under-manned (i.e. you don't have enough troops) or your troops are of insufficient technological dvelopment to attack your enemies (i.i you have cannons and they have marines defending). What you can do in such situationsis using a diplomat or spy, bribe the city to revolt. This is also useful if you are going onto another island and need a safe place to land your troops.
Also having some spare money i useful in case you need to rush buy a unit or wonder (to build it before someone else). I raise this money by not concentrating on science and building markets, banks etc. Also it is important to build lots of roads as they add money as well.
Science can then either be exchanged or stolen, although this means you can never get really ahead of the other civs.