If you file a provisional patent purely to try and protect yourself, wouldn't that document the "invention" sufficient as such to be considered prior art? And if prior art is a valid defense, is it necessary to even file a full patent? In other words...
Is a provisional patent as prior art a sufficient defense from someone else who might assert theirs for the same "invention", provided your provisional filing was filed first?
Documenting an invention in a way that never sees the light of day does not create prior art. A provisional application (it is not any kind of a patent, just an application) is not "published" and, if not followed up by a non-provisional application, never does see the light of day. If you are interested in a defensive publication you would want something that an examiner is likely to see that keeps others from getting a patent in the first place. Having ammunition in your back pocket that might be used in an invalidity defense in case you are sued just changes the probability of who wins in an expensive future legal action.
If you do follow through and file a nonprovisional that cites the provisional and that application gets published or becomes a patent then the provisional will be prior art. (One of the few board of appeals cases that is precedential - ex parte Yamaguchi)
