This is why the LGPL was created, so that you can have modifications done on your library be free-as-in-speech, but still make the library as a whole useable for a wide variety of other projects, including closed-source versions.
Having a separate requirement to email you for a free-as-in-beer license is just overly complicated for this. The more hurdles you put up for people, the fewer that will adapt the library. I think that licensing is one of those cases where is doesn't pay to be clever. Plus, what happens when you decide to stop maintaining the code? Do you want to keep getting emails for licenses years from now?
Edit: in last paragraph, I said free-as-in-speech, but meant beer (see comment below).
Should we stop maintaining the code or get bored mailing free beer licences, we'll very likely change the licence to LGPL or MIT. Until then beer comes via email.
I wish it was simply licensed MIT or BSD, but congratulations on your software and sticking to your convictions.
Is it copyleft in general, or the patent grant that hinders your work in in Academia? I not sure why you should be using other peoples work for free, but then go around and sue anyone who copies or improves on your work.
The project wrote down exactly what they wanted to do with their work on their license page. I say good for them. More people should do so and think what they themselves want.
My stance is that since they did the work, the authors of the library can license it however they'd like. But, if they wanted to get more people using their library, I think that they should rethink their approach. LGPL is more appropriate for a library, where you can still have your copyleft approach for the code you wrote, while still promoting wider use.
Here's an extreme edge case... as they said, if they get tired of supporting the email to get a free-as-in-beer license, they will just open it up with an MIT/BSD style license and be done with it. That's great. But what if someone gets hit by a bus? Or someone leaves the project and moves to Antarctica? There would be no practical way to release an unencumbered version.
Really though, they can do what they want - it's their code. But licensing is one of those areas that you really shouldn't try to be clever.
They actually don’t want as many people as possible using their library. That is not the goal when choosing the GPL. The goal is to maximize the number of free users in the world – that is, users who have the freedoms which define Free Software. Mere users is inconsequential. If users is what you desire, then by all means, choose a permissive license (MIT/BSD/etc).
And yes, getting RMS to change something is quite the accomplishment :) His ability to walk the talk is impressive.