The advantage of using a proper data format is
a) You don't have to do in-band signalling so it will be far more reliable (you still can't have spaces in filenames for a lot of unixy things).
b) The encoding is standard. Using text for pipes still requires some kind of encoding in general, but there are many different ways (is it one entry per line? space separated? are strings quoted? etc.)
In other words you still haven't solved the fundamental problem of passing data back and forth between different programs. In fact if you are going to mandate a specific serialization/deserialization format then JSON, XML, or even ASN.1 are better options than s-expressions.
The other point was that we're ultimately stuck with serial forms of communication, be it wires, pipes, sockets etc. If we want to easily transfer structured data through these serial channels, we should probably build up our structures from a serial primitives, and S-expressions are much more handy than plain strings (which we may not even be able to parse without ambiguity), or XML, JSON or whatnot. One, because the parser is already implemented as part of the language, and secondly, because you can transfer code in addition to data, and evaluate it on the remote end to bring into scope more "structured" data like records.
I did try to include a bit more in the previous post, but I'd accidentally hit save, and I was unable to edit the post afterwards
So even within a controlled environment like PowerShell where everything is a cmdlet it is still impossible to pass only structured objects between commands.