A similar trick is used by Sublime Text. You can use the product without the key indefinitely. But every 10 times you save, it shows a prompt for buying the so

A similar trick is used by Sublime Text. You can use the product without the key indefinitely. But every 10 times you save, it shows a prompt for buying the software. Not even intrusive, you can discard it easily, and be productive for months. But after months of reading that you don't pay, the guilt sets in. After all, you have been enjoying the product, since you have been using it for months.