might as well be a limited benefit plan.
Not exactly.
For starters, each plan is different. Some use a network (such as Multiplan) while others attempt to negotiate fees after the fact.
These operate more like a plan with a very tiny network where you can be balance billed for almost anything.
FWIW, I am not a fan of these plans but I believe the danger is much more sinister than your post indicates.
As KG indicated, they are not insurance. Try scheduling an expensive, elective procedure with a provider when there is absolutely no guarantee they will be paid.