I said next to worthless. Something is always better than nothing (except SpareAir).
If you actually read the document, you'd note that it isn't expected for people to follow for every dive. That is simply impractical. The document is largely a reference material to give the students any tools to give them the knowledge and confidence to plan their own dives and not be dependent on others to do the dive planning for them. Those motivated enough (such as someone making the effort to post on SB), they'll receive some benefit greater than those plastic slates. A number of instructors in the Puget Sound use the document when teaching open water. A number of instructors in different parts of the world do as well.
I'm still going to argue that a plastic slate with a basic checklist that a diver uses every dive to discuss the basic dive plan with their buddy has tremendous value over what the vast majority of divers currently do for dive planning. You might see this kind of slate as too simplistic, but that's why it's effective... because it's simple enough for divers to actually use on every dive.
Imagine how much better diving would be for everyone if all divers spent just two minutes of dive planning with their buddy prior to getting in the water.
I'll also argue that gas planning does not need to be complicated. I scanned through your planning document, and I could imagine most people seeing what is there, along with the accompanying spreadsheet, and being overwhelmed by the idea of having to do complex calculations before every dive. The fact is that gas calculations should be simple enough to do mentally (albeit with a little practice.)
As a physics professor, I spend a good deal of my life trying to convince my students that what I want them to do is not nearly as complicated as the rest of the world has led them to believe. I see the same issue with gas planning calculations.