My take (also posted in the list on 2006-04-12):

I am fine with having the language mandating that steps are not to be executed in parallel. I haven't seen so far use cases that are calling for parallel execution, and I feel we would make the language overly complex by addressing the issue of parallel execution at this point.

If there is a need, a particular engine can implement an extension to support parallel execution and based on that experience we can revisit the question in a subsequent version of language.

I prefer specifying that we should consider this case and "not prevent" an implementor to make such optimizations
If for some points we see that this condition complicates too much the language, we should push it later

Created attachment 418 [details]
Parallel vs. streaming

To avoid a possible confusion, we are talking about parallel execution as illustrated by the first pipeline in the attached diagram, not streaming, which is illustrated in the second pipeline in the attached diagram.

http://www.w3.org/Bugs/Public/attachment.cgi?id=418