David Berube's blog post "How I Quickly Test and Validate Startup Ideas" outlines a ingenious form of concept testing. He achieves something researchers strive for - and frequently compromise on: he gets feedback from a wide audience. Very quickly. To put it in design research terminology, what he's creating is a research stimuli to ask a narrowly framed question, "are you interested in this." Asking for emails sets the threshold for the affirmative response high enough that it serves as some indicator of what people would actually spend money on. Iterating through the process multiple times allows for greater granularity.
The problem is, he's just asking one narrow question. Over and over again. If you spitball 1,000 ideas at the world to see what sticks all you get back is a shallow data set of yes/no's. 1's and 0's. To design and improve his concept, he needs more nuanced data. Yes/No answers may tell him if the concept he is testing has appeal, but they don't say anything about why. They don't help him advance, adapt or pivot.
He only devotes three lonely sentences to it, but David does point out a way to get around this shortfall -- use the email address collected. Through the address you have direct contact with potential future customers. Reach out to them, ask them questions, and you can dig down to understand their needs, desires, and pain points. That is how you advance a design. Getting the email addresses is, I believe the most valuable part of this whole process.
Maybe this is less a form of concept testing, and more a bootstrap method for recruitment.
If only you could capture the emails of those who ALMOST put their email addresses in, but found the solution lacking in some way. Now that would be a rich data source.