They aren't supposed to grant a patent if there isn't a reduction to practice. In practice, the PTO is overworked and things get through that shouldn't.
If it comes down to a lawsuit and there was no reduction ot practice, a patent can be thrown out. Of course, when it gets to court it's very expensive for everybody.
The best way to demonstrate the practicality of a software invention, for instance, is to attach working source code to the patent -- with the funny consequence that a strong software patent involves, in some sense, a contribution to open source.
That's not funny or ironic, that's one of the two things patents are intended to achieve: to encourage inventions to be published for everyone to use freely (after the patent protection expires) rather than kept secret.
it makes sense from the perspective of the patent system and it's goals, but it's contradictory to the "patents bad" attitude that is common in the open source community.
If it comes down to a lawsuit and there was no reduction ot practice, a patent can be thrown out. Of course, when it gets to court it's very expensive for everybody.
The best way to demonstrate the practicality of a software invention, for instance, is to attach working source code to the patent -- with the funny consequence that a strong software patent involves, in some sense, a contribution to open source.