Yes, free market is great if you have corporations (it reduces the cost!).
But when you don't (because you're playing warlords for instance

), mercantilism has its uses.
Short list of situations where you can make good use of mercantilism :
1) Everyone hates you (or runs mercantilism). You can compare a free specialist to 1 domestic trade route. It's pretty rare that the trade route is better than a free specialist.
2) you're the big boy (=more than 30% land). This means that most of your trade routes are going to be domestic anyway. Almost same situation as 1, finally. However, your best trade routes may very well be in your best cities. It's even very likely. So if your best cities are really better than the others (happens often when running for domination, your last cities are fillers more than anything else), you may still be losing out.
3) You're running a SE. It's very likely that you don't have all the buildings in all the cities. Your cities probably aren't ready to use the commerce to its full power. This means that 1 specialist that you can choose brings in more than commerce that you use for culture anyway to keep your big cities happy.
4) you don't care about techs or gold. You want production. A free specialist can be an engineer or a priest, or in any case a citizen (1 hammer is still better than 0).