Right indicator on is a great German technique that is completely ignored in the UK.
right indicator on means you're an arse - I know you're there and I will get out of the way as soon as i can anyway ..... now you're just pissing me off
You only use it if you know they can get out your way and haven't done it for some reason (not paying attention etc.). I wouldn't sit behind you with it on for no reason.
The indicator on while in the outside lane, on the continent, is also used to let drivers that might come up behind you on long empty highways, know that you are aware you are in the overtaking lane and are in fact overtaking (usually at around the speed limit -as opposed to anyone who comes up behind you going faster but also faster than legal).
The first time I drove in France it took me a while to realise why everyone was indicating while overtaking. But it's actually pretty handy because as you said, you know the car is only overtaking and will move back into the next lane as soon as they've passed. A lot better than trying to guess what people on UK roads are going to do next!