This slogan creates a sense of scarcity and it works. People who would otherwise buy 1, or even one, take 3, just to be sure. Google used it to get users to sign up for Google Wave RIP.
The same is happening now with people stocking up with supplies. They see a half empty shelf, better make sure not to be left out, and the shelf empties out completely soon after.
When thinking about how to use scarcity in your presentations, consider the long-term implications to your brand as well. User might feel duped when they find out that you opened the flood gates 3 weeks after that special offer.
In the case of SlideMagic, the early restricted signups were required since things were still buggy. Today I removed the required to sign up for an account before downloading the app as a sign of confidence in the platform.
Photo by Miguel Pinto on Unsplash