Why bother with a flag? If you like it fine, but I wouldn't expect any safety benefit.
I'm always worried a bit by the impatient drivers who wait for you to cross a junction, and then turn across your path, closely behind you. These are the sort of people who don't see a trailer there, since (i) they don't expect to see it and (ii) it's largely behind me, so their view of it is obscured.
There is also the case in heavy traffic, where if I was in a lane next to a motor vehicle, he simply may not be able to see the trailer, since it's below the level of the window, and this is worse if I pull up after him, so he hasn't had a chance to see me when approaching. If he pulls across into what he thinks is an empty space, he may take out the trailer, with consequent crashing issues for me.
Of course, a flag won't necessarily make the trailer that much more obvious, but it only has a small cost in terms of additional drag.