Q/R tucked behind/against the fork and stay front & back.
Yes, maybe as you're riding along nothing's going to catch on it (unlike on the MTB where you're brushing through branches and nettles)
And maybe you are careful what you lean it against in the garage/shed or when you park it up.
But if you're parked at the caff or somewhere and some silly sod leans their bike against yours, interlocks bits of bike, then removes theirs by yanking it away from yours...I don't want to risk them pulling the Q/R open thank-you very much.
Apparently the Q/R is more aero when horizontal but I think I can live with the, ooh enormous, drag of it snagged-up against the fork
- especially when I'm racing in a Tri, when I've racked my bike on a scaffolding-pole with lots of other bikes belonging to clumsy & hasty transitioning triathletes, and it's pretty likely the berk next to me will be yanking and tugging to disconnect his bike from mine !