Really can't see any advantages to a trailer for on-road touring.
on-road is whole advantage. Trick is to get the trailer right. One wheel trailers are better suited for off-road, two wheel trailers are better for on-road.
The key to trailers is they separate the influence of the load from the bike - you put more energy than you'd imagine into balancing the loaded bike, not just propelling the load along. One wheel trailers still need to be balanced by the rider, so they're not really that much more efficient than panniers, but two wheel trailers don't. Two wheel trailers (so long as they're hitched at the dropout and not somewhere higher up like the seatpost) don't influence the bike at all. Its the removal of that balancing effort that makes the difference - you'll either travel measureably further for the effort, or move much more weight for the same effort with a 2 wheel trailer compared to panniers, by quite a large margin. Furthermore you can load them as clumsily as you like, you don't need to spread or balance the load. Other than up hill you forget its there, your bike feels like your bike, it doesn't feel heavy or loaded (until you go through a narrow gap).
The downside as you suspect is that its an extra item to deal with getting yourself on/off of public transport. French trains are ace for cyclists once you're on them, but getting around the station and on and off the train can be a bit of a hassle, less so in some stations, but Nice for instance is like the bloody royal tornament, I can't image there are any wheelchair users in french stations, or anyone elderly getting on and off the trains - in some stations you climb up into them through narrow doors.
While a trailers is extra encumberance in this situation again 2 wheel ones are better, they unhitch easier and good ones balance well in the hand so you can wheel the bike with one hand, and trolly the trailer along with the other. With a one-wheel trailer you've not really got an easy way of dealing with it once its loaded, but off the bike.