There are two variants of MathML: semantic MathML (which does capture enough structure of an equation, and sees very very little use) and presentational MathML (which doesn't, but is much easier to author, and accounts for almost all MathML usage). There's no real difference in usefulness between presentational MathML and TeX.
The verbosity of MathML makes it completely impractical to type as working mathematician. It’s not just a small difference; it’s completely ridiculous.
