If you join 2 wires together how are you going to select main beam? you may never need it but it will fail an MOT if it doesn't work.
Most likely a faulty wire in the headlight switch or a broken contact plate, fix it again or buy a new switch is my advice.
The 100N never had a good charging circuit, running with the lights on is ok but if they were permanently on and you ran into a problem you'd not get the bike home, if you can switch the lights off at least you can save power when it's needed and of course if it started to blow fuses and the lights were at fault you'd struggle to isolate the offending part.
Modern bikes have a circuit where when the bikes trying to start the headlight goes out, this is to allow the engine the maximum power in the battery so it can start.