Generally speaking, Protestants often argue against Roman Catholic doctrines that cannot be demonstrated by scripture alone or exist on the basis of papal authority. And yet, many Protestants accept the doctrine of the filioque in their statements of faith and ordination rituals.
The filioque has a weak scriptural base, if any at all. It doesn’t have much if any serious patristic support until the fifth century and then it is quite limited as being purely a speculation on Augustine’s part. And it doesn’t begin to become enshrined in major doctrinal formularies until begining in the 11th century. And it is responsible in part for the largest schism in the history of Christianity, not to mention an alteration of the major doctinal statement ultimately on the basis of claims to papal supremacy.
The argument is that if we assume Sola Scriptura and assume argumentation along this line works for the assumption of Mary, why doesn't it also work for creation out of nothing or the filioque?
Read the rest of the arugment here: http://energeticprocession.wordpress.com/2007/08/25/an-apologetic-two-fer/
I've read similar arguments along this vein before, mostly dealing with the canon of scripture or why the Sabbath is on Sunday instead of Saturday. I've also seen quite a bit of argumentation about how scriptural the doctrine of the Trinity is. I think this is the first I've seen that's significant enough to get the point across and far too difficult to rely on scripture alone.