Your comment got me thinking about that, especially as I currently very rarely use the Flexible Cover, if at all, whereas with my previous vehicles I used it all the time.
I guess the main reason for having one is that they come as standard in the UK, and all my vehicles over the last 30 years or so have had one, so it probably boils down to force of habit. To be fair, the rear parcel shelf on my hatchbacks lifted out of the way with the tailgate, so it was only with my estate with the roller blind style cover that I had to make a physical effort to release or pull into place.
It is general advice in the UK that if you have to carry any valuables in your vehicle to leave them in the boot as the parcel shelf stops people seeing what is in there. I also used the rear parcel shelf for placing my dhobi gear when having a bird bath at the side of the road when traveling through Spain and France. I used to dry my towel and facecloth by spreading it out over it as well.
It will be interesting to see if I get any comments about not having the 'correct' rear parcel shelf when it comes to trade in time, though I'm hoping that is still a considerable time away!