What an terrible syntax and that for a rather poor result. The comparison table betwenn TeX and MathML shows how far MathML is from TeX (for expert eyes the MathML rendering is awful almost like à la Word). Obviously MathML is not close to improve on the work of Donald Knuth.
The given example
<msup> <mi>x</mi> <mn>2</mn></msup>
is expressed in TeX (switching from text mode) as
which is identical (except for the $'s) to the convention of most computer algebra languages (Maxima, Maple, etc.) which don't need a special notation to know about the functional meaning (even bulkier in the given example).
I get the impression that once more some people need to reinvent the wheel, and dismiss the past experience, resulting afters years of work in a big failure IMHO.