First of all, peace, brother. Not trying to interrogate you or pick an argument. Trying to keep anyone who uses a 3 pt hoe aware of some inherent safety considerations. If I yanked your chain in some way trying to do that, well, my bad.
Here are some examples of accidents that resulted from either breakage or the failure to properly secure the linkages on a 3 pt. hoe:
http://www.tractorbynet.com/forums/safety/40074-3ph-backhoe-limiter-safety-bar.html
http://www.tractorbynet.com/forums/safety/26166-death-back-hoe-canopy.html
mounted back hoe
Beyond that, everything in my experience has pointed to that toplink being under additional stress with a 3 pt hoe. In fact, some of the tractor manufacturers actually offer reinforcing kits for the toplink attachment points on their equipment for use with 3 pt hoes.
Here's a question for you: If there's very little force going to your toplink and the stabs are carrying the load, you could probably operate the backhoe without it, right? Do you think you could lift the boom off the ground if the toplink wasn't attached? What about with a full bucket? Could you easily remove a good sized rock? I'm willing to learn here, but the way I see it, your lifting and curling power requires both the stabs and the toplink to get any leverage.
I've looked at some of the Youtube vids of that Northern Tool unit and a few others like that. What I saw mostly confirms my point... any time the operator tried to lift a seriously heavy scoop vertically, the road wheels came up off the ground because there was no leverage.
The world won't end if we disagree about that toplink, but I'm sure we can agree that any equipment - backhoe or what have you - needs to be operated safely. Cheers.