Basically, you would attempt to steer trade power (TP) back to the node in which you have your capital, as that is where you will potentially collect the most. You would use your light ships to add trade power to the nodes, so as much as possible gets steered in the desired direction.
However, in practice it is not so simple. It is possible to have high TP in a node, with no merchant, and still benefit from it. Let's break it down.
Trade power can be used to either steer trade value, or to collect. If you have no merchant present, the default mode is to steer trade away from the node (exception: you always collect in the node in which you have your capital).
Thus, as you add more ships, more value will leave the node. However, since you have no merchant present, you have no power over where that trade is going.
If no nation has a merchant present, the trade will be evenly divided between all outgoing routes. If any nation decides to add a merchant to influence the direction of the trade, all outgoing trade is going in the direction they choose. If the trade is already going where you want, you can add more ships to ensure more value is transported your way. No merchant needed!
Practical example from my own game (with made-up numbers)
I am Norway, my home node is North Sea, and I automatically collect there. I have two merchants. I also control the coast of Eastern America through a subject nation (Vinland).
The Caribbean has 20 ducats in trade value and two outgoing routes. Cuba has high trade power, and collects 100% of the trade value. I transfer ships to match their trade power. Now 50% (10 ducats) is going out of Cuba, even if I have no merchant present.
Vinland prefers that trade to go to Chesapeake, where they are collecting, so they send a merchant to pull trade in that direction. 100% of those 10 ducats are thus transported forward to Chesapeake, even if they add no ships of their own – my massive trade power is making sure value is being transported out, they only influence the direction.
I don't want the trade to stay in Chesapeake, however, so I add ships (trade power) to that node, so the value will be going out once more. Portugal wants the same, so they also add trade power. Our combined trade power matches Vinland 9:1, thus 9 ducats will leave Chesapeake, and Vinland will only collect 1 ducats out of the incoming 10.
However, the Portuguese add a merchant to steer all that trade to Western Europe. No bueno! I also add a merchant, to steer the trade further to Gulf of St. Lawrence. Let's say I have double their trade power, so out of the 9 ducats, 6 will be transported to Gulf of St. Lawrence, while only 3 make it to Western Europe.
In Gulf of St. Lawrence, pretty much everyone agrees the trade can not stay there. Even if some Indian native nations try to collect, Portugal and I both add all our remaining ships, so no trade is staying in the node.
There are two outgoing routes, so 3 ducats are going to the North Sea, and 3 to Western Europe. However, as I add my second merchant, I can steer that trade in the direction I want, and since Portugal have no merchant to influence the direction of the trade, everything is going to the North Sea, where I collect everything.
Haha, stupid Portuguese
If Portugal wants in on the cake, they need to add a merchant. Right now, I am the only one of us benefiting from their light ships, thus if they have no merchant to send, they will probably send their ships somewhere else.
Oh, cool, so this will always work, right?
In my actual game, I managed to transfer 17 ducats from the Carribean to the North Sea. Pretty good, especially considering I owned almost every province in the North Sea trade node and thus could collect nearly all of it. However, all the other European jerks decided Norway doesn't need money, so they added a whole lot of light ships, to make sure the trade value would leave the node again, so all my efforts was for no gain. It actually made more sense to collect the value in North America.
However, some nodes, like Venice have no outgoing routes – 100% of the incoming trade is collected! If you dominate that trade node, and manage to steer a lot of trade into it, there is no way to transfer value out (well, a minimal amount can be transported upsteam, but it's insignificant).
To answer your question: No. Sending light ships to a node in which you have no merchant is not wasted. Adding light ships will make sure more trade leave that node. This is beneficient, if you know where that trade is actually going. In my first example above, Vinland was pulling trade in my desired direction, but had very little trade power. When I added my merchant fleet, it had a huge effect. I didn't have to add a merchant to transfer trade power – that is done automatically when I add my ships, Vinland only affects the direction.
Also remember, that you can use this power for evil. If one of your enemies is relying on trade, you can basically shut down their income by making sure all trade is leaving the node they are trying to collect. If you add a lot of ships, the trade will leave the node, and they will earn less money. If you care about where the money is going, you may need a merchant to steer the trade.