<i>&gt; It makes the library unusable for anyone, but open source software.</i><p>Well, the author is free to dual-license it to propriertary software for some fee - if they want.<p><i>&gt; I don&#x27;t think that&#x27;s a good thing for promoting a library&#x27;s use</i><p>But it&#x27;s a good thing for promoting Open Source!<p>That&#x27;s the tactic question here: Do you want to promote your name and your library, or do you want to encourage more people to open source their stuff? In general you achieve both, but the question is: Which aspect is more important to you?<p><i>&gt; If you want your library to popular don&#x27;t use GPL. What happens is that someone else comes a long and creates a MIT version.</i><p>See <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10158535" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10158535</a>