The reason is simple: between him and the other candidate for the ballot’s top position, Christian is the only one funding my keynoting and evangelizing
So even among the Pirates, is it always about the money? This has not garnered too positive a response, especially in the light of the fact that the "other candidate" is Amelia Andersdotter who, as some would argue, does a better job. The heat is on, including a twitter conversation (and a Diaspora hashtag for the decentralites among you).
Another possibility would be dual-licensing with CC-BY-SA and GFDL, but that's probably not worth the extra work unless you've identified materials you want to use that are under GFDL.
If the things they want to use are GFDL-only and the product is an adaptation of those then they don't have the option of dual licensing. You may have meant "you want your work to be incorporated into that are under GFDL."
Only do CC-BY if you simply want to make a gift to the world, and you don't care if your work is repackaged into something non-free by other people.
CC-BY isn't quite a pure gift -- it could be used by a selfish licensor if that person only cares about maximizing the amount of credit they get -- incorporating CC-BY works into non-free works still requires giving credit.
Some programming languages manage to absorb change, but withstand progress. -- Epigrams in Programming, ACM SIGPLAN Sept. 1982