I'm not sure of "more than a couple of sentences" being the guidance for providing a long text alternative. I've always understood it to be if it requires more than about 100 words, OR if there is a necessity to structure it, then a long and structured description should be provided. A couple of sentences means about 20 words. Do we really want people to start requiring a long description if the alt is more than 20 words? Remember, the general public will take this document as the final word... I would like other peoples thoughts on this.