An Atom feed is going to be defined as an XML document, which means that if it's not well-formed then it's not Atom. All it needs is for one (I repeat, one) popular newsreader with a large installed base to enforce this policy (stop parsing and display an error to the subscriber) to turn this from de jure to de facto reality. This works because Atom doesn't have an installed base.
After wrestling for days trying to write Java code to read Chef Moz RDF database files, XML files in UTF-8 format that turn out to be neither UTF-8 nor XML, I'm in strong support of strict parsing for new formats. Without it, there's no penalty for being non-compliant, which leads to lots of bad data supported by workarounds that have to be dealt with forever.