From OFLB

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Reasons for a purely OFL Open Font Library

1. A single licensing option implies clarity of vision for the project; leadership.
2. While this discussion has been going on, Ed Trager has released his 'Go for OFL!' campaign - http://www.unifont.org/go_for_ofl/ - which we would do well to support, especially because of...
3. ...the coincidental namespace - Open Font Li* - is a bonus - people will remember one from the other, if their link is emphasised (a good memetic trait, worth exploiting)
4. OFL is designed to be attractive for designers and free software advocates; PD and even GPL are notoriously unattractive for designers.
5. Since currently the only OFL fonts are high quality, requiring OFL will set a bar of typographic quality
6. Other websites already list Free Fonts under a variety of licenses
7. The Free Font Movement lacks a center for a development community. The OFLB can become that center! But it needs...
8. ...License Solidarity. License proliferation retards development, and the wider Free Software movement is heading for more license solidariry with GPLv3 and Sun's recent substantial GPLv2 contributions.
9. There are few substantial fonts released in the Public Domain already, and PD status means its easy to relicense them as OFL to fall into line.

But I have a Free Font not licensed under the OFL?

To contribute to the OFLB, please switch licenses to the OFL.

Since OFL fonts can be considered "free, as in freedom", this won't take away any essential freedoms that your fonts already have with their current license. There is really no disadvantage in doing this, other than the work required to rebuild a package with a new license.

If it is important for you to retain your existing license, such as the GPL, dual-licensing is certainly an option. This doesn't remove a single iota of freedom from your fonts, because the existing version lives on, with all freedoms intact. But it makes them also compatible with other OFL projects.

However, by dual licensing, you may find yourself left out of the OFL commons. That is, improvements may be made to your font and published only under the OFL. You will then be unable to fold those improvements back into your own font under both licenses.

Of course, if a font is in the GPL commons, there is absolutely nothing that can remove it from GPL-space, and retroactively take away freedoms intended by the author. It just might become obsolete over time.

Therefore switching is the best option.