Can anyone explain the logic behind dipped headlights turning off when full beam is activated?

I can't think of a scenario or situation whereby I would be on high beam and NOT want to see the road immediately in front of me.

I assume my FL1 is not a rogue in this respect and that it does it on them all? It's just annoying me!

it means it is less likely to set fire to your car.

If the bulbs are twin filament H4 bulbs, the dips turn off to avoid overheating. The bulb life is seriously reduced if both filaments are on for long periods of time. The heat generated would likely melt the headlamp plastics too.
In the facelift freelander, dips remain on when the main beams are on. The facelift uses separate H7 bulbs for dip and main so excessive heat in one bulb isn't an issue.