Just to make it GPL on pure ideological grounds would seem pretty pointless. but the GPL is also practical.
A GPL driver would allow the use of the 2.6 kernel, which would have a lot of benefits, the biggest probably being battery life. Most people won't want to use 2.6 if it means giving up the SD slot.
It would also allow using MMC cards bigger than 1GB, they are getting pretty cheap these days and I would like to upgrade mine. It is a shame to have to stick with 1GB because of shortsighted programming in a binary only module, especially in linux.