When mIRC 7.32 encounters a character that is unsupported in the current font, it displays the necessary glyph from an arbitrarily chosen font that does support that character. Unfortunately, as this fallback font cannot be manually set, problems with legibility can arise in channels where languages are mixed.
Take Chinese or Japanese, for example. Being languages that use Chinese characters for their writing systems, supporting fonts usually need to be set at higher point sizes to ensure legibility. Only a few, such as MS Gothic, can be viewed at smaller point sizes without difficulty. However, those handful of legible fonts will only be selected by mIRC in the situation where there are no other fonts installed on the system which would be picked in place of them. And if one were to simply increase the font size, then the English text would become too big and unwieldy to read comfortably.
Now, one could bypass the issue of Japanese or Chinese legibility by simply using a font designed for those languages (such as MS Gothic), but therein lies another problem. Those fonts are designed specifically for those languages, and not English. So the English ends up looking ugly. Try setting the font to MS Gothic or SimSun to see what I mean.
If we had selectable font fallback, all of these problems can be solved. We could choose an English font that's easy on the eyes, such as Tahoma, while having a fallback font that's nice for that particular language. The best of both worlds.