Living languages and writing systems evolve.
Using the combining low line to show stress seems reasonable to me,
perhaps because it was a typewriting convention I'm old enough to
remember. People unfamiliar with that convention should be able to
figure out what's up from the c̲o̲n̲t̲e̲x̲t̲. Drawing a line under a
word or a phrase certainly draws attention to it!
(Apparently there's a recently evolved practice to use periods between
words. To. Add. Emphasis. Almost as if one is speaking v-e-r-y
s-l-o-w-l-y in order to make a point.)
End users probably consider the entire Unicode set to be their tool
kit. I've seen plain text screen names in both cursive and fraktur,
thanks to the math alphanumerics. The carefree user community seems
unconcerned with the technical insistence that *those* characters should
only be used in formulae.
If, for example, 𝓒𝓵𝓮𝓸𝓹𝓪𝓽𝓻𝓪 𝓟𝓮𝓹𝓹𝓮𝓻𝓭𝓪𝔂 can input her
screen name in cursive, there's nothing stopping me from using
𝑖𝑡𝑎𝑙𝑖𝑐𝑠, if I'm so inclined.
Making recommendations for the post processing of strings containing the
combining low line strikes me as being outside the scope of Unicode,
though. Some users might prefer that such strings be rendered in *bold*
and other users might prefer /italics/. This user would prefer that
combining low line always be rendered as combining low line.
