The idea of a Supreme God governing angels, demons, etc... is held dear by billions of people worldwide, so while the books themselves may not reflect modern advances in technology, the "paradigm" behind them is as vibrant as ever.
No matter what background a magical spell or ceremony has, if the procedure is defined you can try it and see what happens. The grimoires have an advantage (for practicing occultists, at least) in that the ceremonies are described for use rather than for study or mystic illumination. The spells and conjurations of the grimoires, and many other magical texts, do not lose efficacy with time.
The proof of a magical system is found in what you do with it, not in its relation to science or philosophy. Sure you can "make up" anything and use it, but to what extent? Like the grimoires and any other written occult work, the only way to know if you have something useful is to use it.
If you get the sort of results you want, that's great, but are there results you desire that you can't produce? A lot of these desirable effects are listed as the rewards and powers of conjuring in "musty old books," and I think they're worth exploring for that reason. Additionally I try to approach magic with the scientific method in mind, and I never though it was scientific to dismiss an entire subject without at least giving it a fair test.
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