iTunes Plus, Play, Amazon (when it gets to the UK), fine. But with DRMed content, even from a popular service like iTunes, you never know what's around the corner. Your music could just disappear.
A simple rule for good web sites: content is golden. In an accessibility context, structured content is golden. It a core responsibility of the site owner to ensure this is followed (not just the developer). When you examine this issue, you can then understand why few organisations will ever produce accessible PDFs.