(Long) MathML is purely a descriptive language. There are programs that can convert this into graphic equations, but InDesign is not one of them. MathML is "just" a
language -- you can draw the square root of 2 by typing in (presumably) "< sqrt >2< /sqrt >". Now how can InDesign "know" the square root character should be drawn over the 2? It can't -- it's not built in.
The standard XML translation is: everything is either a paragraph, or a character style. Now
two items can be translated directly into an InDesign document. Compare this to, for example, an Excel file, written out as XML. Excel can read it back in and you can do your spreadsheet calculations with it. You can import it into InDesign, but suddenly the cell text "sum(a1..a5)" appears just like that, not anymore as a sum.
Pointing out MathML == XML is not a valid reasoning. I can think of a variety of data written in XML, that ID doesn't process 'natively'. It doesn't process them at all, period.