Mongolian ruby requirements

[r12a]

Because the shapes of traditional mongolian letters often use the same forms, ruby is needed in Mongolian too.

Here is an example, marked up in a couple of different ways, to test joining behaviour.

<p><ruby><rb>ᠮ</rb><rt></rt><rb>ᠣ</rb><rt>o</rt><rb>ᠩ</rb><rt>ng</rt><rb>ᠤ</rb><rt>u</rt><rb>ᠯ</rb><rt></rt></ruby> ᠪᠢᠴᠢ<ruby><rb>ᠭ</rb><rt>g</rt></ruby></p>

I'd appreciate feedback on how often and in what circumstances ruby is needed for Mongolian. I can see that it would be very helpful for those learning to write the traditional Mongolian script. Is it regularly used in this way? Is it used in other situations?

(Thanks to SiqinBilige for the initial heads-up and the example.)