If you don't publicly pre-commit to what you're going to measure then p-values become a bit meaningless since nobody can know if that was the only thing you measured.
If researchers are well organized then pre-reg should be almost free. On the other hand if they're disorganized, winging it and making things up as they go along then pre-reg will look like a terrible burden since it forces them to decide what they're actually going to do.
The short general version of my argument is: feedback > filtering
I would agree that preregistration is one way to make p-values more useful. They may be the best way to determine what the researcher originally intended to measure, but they're not the only way to know if that was the only thing a researcher measured. I've found asking questions often works.
If we're talking strictly about properly run RCTs, then I would agree, preregistration is close to free relatively speaking. But that's because a properly conducted RCT is such a big undertaking tha...
John Ioannidis has written a very insightful and entertaining article about the current state of the movement which calls itself "Evidence-Based Medicine". The paper is available ahead of print at http://www.jclinepi.com/article/S0895-4356(16)00147-5/pdf.
As far as I can tell there is currently no paywall, that may change later, send me an e-mail if you are unable to access it.
Retractionwatch interviews John about the paper here: http://retractionwatch.com/2016/03/16/evidence-based-medicine-has-been-hijacked-a-confession-from-john-ioannidis/
(Full disclosure: John Ioannidis is a co-director of the Meta-Research Innovation Center at Stanford (METRICS), where I am an employee. I am posting this not in an effort to promote METRICS, but because I believe the links will be of interest to the community)