> How useful is it to standardize hieroglyphics, ancient greek musical notation, and emojis as standard text characters esp. without standardizing their screen representation?
In the same way it's useful to standardize letters in various alphabets without standardizing their screen representation. There is semantic content associated with each of these symbols that persists even if there is significant variation in how they are presented. Of the ones you list, emojis are the only ones where this is any more a problematic approach than it is for letters in various alphabets. And as people who don't approve of Unicode adding emojis like to point out, emojis aren't that critical so having some loss in the translation isn't a huge deal.
Remember that before emoji standardization various cell phone manufacturers (particularly in Japan if I remember correctly) started using codepoints for whatever they pleased. The alternative to Unicode not standardizing them was to have a repeat of the OEM font gold rush in the SMP.
> styled math letters (which would have been equally well served by simply rendering them in italics or in a special math font)
That was my first reaction as well, but there are a few problems with that approach:
* Math italic characters look very different from normal italics, and are shaped and kerned very differently because they are commonly used for single-letter variables which will be juxtaposed together in expressions. If your goal is to be able to preserve some math formulas in a purely line based text format, preserving this aspect makes a big difference in readability.
* Many of the math letters and "letter-like symbols" have associated semantic content (like bold for vectors), which it makes sense to preserve. MathML alleviates this to a significant degree but I don't believe these codepoints were intended only for MathML usage.
* On the technical side, OpenType math fonts need to carry associated metadata for many of these characters. Putting them in separate fonts complicates this, since these tables need to refer to glyphs (general codepoints are unsuitable in a number of cases) and each font file would have a different glyph address space.