Partly useful, partly not
Multiple markup is (easily) possible
Using this toolset, it's very easy to create markup which refers to the same part of a document, even overlapping parts, with a minimum of effort. The sameness of those references is generally lost on 'remeshing' to regular XML, but it may be adequate for a lot of tasks.
Handy text-only alternative to XInclude, entities
While Ool isn't designed to be a standard by any means, I've already found it useful for incorporating literal XML documents into existing XML documents without the mess of CDATA or entity references. It makes no claim to XInclude's universality, and its processing model is clear: it only ever works if you're doing Ool processing.
Parallel markup still difficult
Separating markup from content is fine, if and only if the content itself is actually stable. Any changes to the content, of course, mess up the counts. There may be ways of avoiding this through additional software or more robust pointer techniques, but right now the approach works pretty much only with stable content.
Attributes and mixed content are tricky
Attributes are impossible to represent this way without a complex set of conventions, and mixed content produces some rather verbose results. (It requires element placeholders for text, not just attributes.)
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