Hello woppy101...I too have many questions about the reserve seat....

The economy reserve seat I have only seen offered on (some) flights to Australia (they may have them on USA flights also...does anybody know?).
I flew to Australia in Y in May on the DOH-MEL (77W) and purchased the reserve seat for Euro 152.70, Because of platinum I could select the "preferred row" 17 and took the window seat K with the reserve seat being J. I booked it online about 4 days before the flight.
A lady was seated in 17 H and the middle seat did stay open.
I did tell her I had purchased the seat as I was kinda lounging half on the middle seat with the arm rest up and offered her to put her bag/pillow/blanket in the middle seat to get it off the floor.
If I recall correctly there was a note on my boarding pass also of the reserve seat.
The flight was (surprsingly) not completely full (30 open seats) and that is probably why it was offered. I think it was worth it (especially compared to the J upgrade offer of Euro 3200 one way) to spread out a little and only having to bother 1 person to use the lavatory compared to 2 if all 3 seats are occupied.

On the return I flew the A380 in Y in the exit row on the upper deck mini cabin from Perth to Doha and the reserve seat was not offered. There was exactly 1 Y seat open on the plane as per Expertflyer.

Anybody has more experience or knowledge about what flights this is offered on etc?? Is it offered only to status holders or to all passengers?
Does QR only offer this if there is more than X seats available? What if you have purchased the reserve seat and then for some reason the flight get oversold last minute and they need the seat to put a pax in? Will you get your money back?

Lots of questions.....but in short....it was worth it.