Since inline scripts are immediately available, it seems that the only effect the async attribute has on them is that document.write() blows away the document instead of writing to the stream.

While this would allow an optimization in speculative parsing, it seems dangerous to introduce a situation where people developing on an older browser could blow away the document without any other benefit than a timing optimization. Furthermore, if one wanted to deliberately promise not to use document.write() but one otherwise wanted the usual scripting semantics, it would be less confusing to have an attribute like "nodocumentwrite" which would make a parser with speculation capability not start a new speculation. (I'm not actually suggesting such an attribute here, because I don't have data about the perf effect of the potential optimization.)

Thus, even if a minor opportunity for optimization is lost, it would seem prudent to ignore async on inline scripts like defer is ignored on inline scripts.