I think the point about invariant sections is not good if talking about Wikipedia, which has already "GFDL without invariant sections or front/back covers" license.
I think the main reason is that it's rather difficult to use eg. one article of Wikipedia properly, since the whole GFDL text should theoretically be included somewhere too (if it's a printed thing). In the case of a whole book of Wikipedia content that's not a problem, but in case of one article among eg. newspaper it is.
So just that the libre encyclopedia could achieve its goals better (spread information freely in copyleft style), it's useful to have a little less "attribution overhead". FSF perhaps has agreed that GFDL is better for user manuals etc., whole products, instead of snippets of information like articles in Wikipedia.