XML is faster and far simpler to parse than TeX. To the extent that you need to (if for whatever reason you don't want to rely on a LaTeX to MathML or Ascii to MathML converter) you can make the quadratic equation MathML slightly more readable, by not using hex entities, but unicode for − and ±, and the named entity for ⁢.[0] Furthermore, you (and I!) are just far more familiar with TeX, which makes the comparison in readability not particularly fair. Finally, much of the invisible, seemingly redundant mark-up, such as ⁢ or ⁡, can help you avoid some of TeX's ambiguities — e.g. is $ f(a+x) $ the function $f$ acting on $(a+x)$ or $f$ multiplying $(a+x)$?[1] If you were to omit this mark-up (and if you're converting from TeX to MathML and don't want your converter to engage in guesswork, you have to) the MathML would be even simpler.
Using the same format for equations as for the rest of the document (i.e. HTML/XML) is advantageous (in addition to the parsing benefits). In particular, you can use the same mechanisms for styling and transforming elements, as you can for the whole document. For instance, you could easily style parts of an equation, provide pop-ups that explain what each symbol means, when you hover over it, or interactively change the equation. (Much of this hasn't actually been done, outside experiments, because only Firefox properly(-ish) supports MathML, so it would have been wasted effort.)
[0] https://gist.github.com/aplaice/266b092bc48afbbdd46cdbd0ca81...
[1] Presentation MathML is still obviously not semantic, but it can be better in this respect than default TeX — there have been proposals for semantic TeX, but none of them have really caught on.
