Many specific perks regarding Mendeley's advances are given in other comments. A different but related issue is that many of us in the open source community are very principled. I'm sure I'm not the only one who feels a little hypocritical every time I use Mendeley. It's as if I'm using/endorsing a product that supports something unethical, like a store that abuses its employees, a meat market that abuses its animals, or a business that donates to a corrupt state. While Mendeley doesn't strike me as such a harmful business, it participates in the proprietary system that many of us believe is harmful. Yet Mendeley is a perfect example of something that shouldn't be proprietary.

A large segment of Mendeley's target audience is part of the open source community. That many of us feel strongly about this issue is clear by the other comments and number of votes about this suggestion, not to mention correlated ones (regarding BibTeX and DjVu support, for instance).

Back to the issue of Mendeley's advancement, the votes and comments here don't represent the degree to which going GPL or open source would be beneficial. That's because many folks in the open source community aren't represented here due to the very fact that Mendeley isn't open source. I wonder how many individuals use or develop open source reference managers like JabRef but would instead use or develop Mendeley if it were open source. And I wonder how many of us who now use and would like to develop Mendeley will switch as those competitors come to meet more of our needs. I've given reason to believe that these numbers are quite large. There's a lot at stake here---even more than the obvious figures indicate.