You didn't ask me, but I'll throw my thoughts out there anyway.
I had a couple of Retrax covers on past trucks. Then I had a BakFlip. Then I had a Bak Revolver X4.
I am currently trying to decide what to put on my new truck, but I am leaning towards another Revolver.
The two things I don't like about the Retrax are how much room the box takes up, and the fact that it needs a separate key. Oh, and also, how trash (leaves, etc) can get down inside the box, and how the drain tubes come out of the box and go through drain holes in the bed. Mine were always getting knocked loose by stuff I would put in the bed.
I liked how with the BakFlip or the Revolver, all I have to do to secure my bed is lock the truck with the cover closed. Having a separate locking arrangement for a Retrax would mean I absolutely KNOW there would be times I would be too lazy (or forget) to lock the cover itself.
I like the Revolver best (of what I have had) because it takes up no room in the bed, open or closed. And it does not block my view out of the back when it's rolled up. And, FWIW, I like the looks of it better than the others I have had. And it's easy to take off and light and compact (compared to the other options) to move around or store when it is off the truck.
I'm just curious, what kinds of situations do you have where you find the ability to lock the Retrax in an intermediate position to be useful?
I had Retrax covers for years and I thought that concept sounded cool. But, in real life, I never did use it very much at all. I also found that it was finicky. The tension that it would clamp in place meant I often found that the cover moved after some driving with moderate braking or acceleration. Adjusting the clamping tension to get it where it would not move made it kind of hard to close.
Then I switched to a BakFlip and then a Revolver. I have never missed that ability to have my cover locked in a partway open position. Though I will admit to driving once or twice with something like lumber sticking up out of the back and the BakFlip being closed except for the last panel just resting on top of the lumber.