But anyway ... if it's not useful it doesn't matter that it's patented and if it is then you've got full disclosure. So I'm not sure the percentage is really important. Perhaps the fees should increase at a greater rate to encourage early release of patented inventions that aren't high worth.
Personally I'd limit the term to about 8 years for all patents and dispense with US business method and pure software patents.
I'm having trouble thinking of something that:
1) You can benefit more from as an industrial secret than as a patented invention.
2) Society is harmed by the lack of public disclosure.
Of course I cannot give any specific examples. They are currently secrets, or they are secrets that died with the company or died with the obsolescence of the product.
The problem then is that there is little to no incentive for the applicant to share the idea. If it's something no one will come up with, then you can keep it secret and benefit from an indefinitely extended monopoly.
The quid pro quo of patents is early information about new inventions in exchange for limited time monopoly. Here you're encouraging inventors/companies to prefer industrial secrets.
If inventions meet your requirements but are kept secret then the public domain will be worse off as no one gets the benefit of being able to privately use/research the invention unless it can be replicated. With a patent the full details are disclosed.
Obviousness is very difficult to assess because some things that took many years developing are so ingenious that they seem obvious. The applicant always has the argument - if it were so obvious how come, in such a well worked field, with such demand for this invention, how come it wasn't already developed? The answer of course is that it wasn't "obvious" [to the skilled practitioner with knowledge of the prior art in the relevant domain].