I have never seen a rock rake with a flexible center link. However, I can assure you that my hyd toplink fixes that problem quite nicely. Without the wheel kit, it would not be necessary. I shorten my top link for the main usage to get the most dig, then I lengthen it to run mostly on the wheels for a final dressing.
I don't think the wheels would do their job as well without a rigid top link.
Every time the horizontal attitude of the tractor changes, the depth of the rake would change as it relates to the wheels.
I.E. if the nose of the tractor was to go up in relation the the rear tires of the tractor, the ends of the lower 3pt links would drop...
With a flexible top link, this would allow the front of the rake to drop in relation to the rake wheels, causing it to dig...as the wheels fall into the "dug" the rake would dig even deeper
With a rigid top link, the relationship of the rake teeth depth to the rake wheels would remain the same(within reason), as the top link would keep the geometry the same.
I use the rigid(adjustable) top link to set the depth (aggressiveness) of the rake... I usually set the wheels at one spot, then use the top link to fine tune the depth.
I'm going to say I much sooner have a flexible top link between the tractor and the rake. Then you can raise the 3 point arms and rock the rake back onto the wheels thus lifting the teeth off the ground an inch or so. Now the rake will only take off the high spots and fill in the low ones regardless of what the tractor is running over.
Wouldn't be without it on a landscape rake.
I discovered how nice it worked on the rake by accident. I made the little 4" extension after I installed Pat's Quick Hitch which extended the lift arms 4 inches, I didn't like the idea of having my center link screwed out so far. It wasn't until I hooked the rake up that I had that eureka moment.
Everything Attachments does that same thing on their Extreme rakes by providing a slot.