Typesetting using MathJax (including mhchem) should not be an end in itself but should be a value-adding activity.
As long as the individual expressions are correct and in accordance with the printing rules, it doesn’t matter whether the formatting is implemented using markup, HTML, Unicode, or MathJax.
NaOH
ρ = m/V
c = 1 mol/l
$\ce{NaOH}$
$\rho=m/V$
$c=1\ \mathrm{mol/l}$
It is true though that typesetting using MathJax offers more possibilities and that many ordinary expressions cannot be correctly set without using MathJax.
$\mathrm{SO_4^{2-}}$
However, simply surrounding any (flawless) text expression with dollar signs ($) does not automatically improve the formatting, but it can easily introduce new errors.
1 kg
$1 kg$ (wrong!)
And of course, a wrong expression cannot be magically healed by mere conversion to MathJax.
Furthermore, MathJax has its disadvantages. The expressions render slowly and are disruptive to comfortable reading (not to say they are a typographic disaster). In some rare cases, it is even impossible to correctly set an expression using the given features of MathJax.
μm (micrometre)
δf (infinitesimal variation of f)
$\mathrm{\mu m}$ (wrong!)
$\mathrm \delta f$ (wrong!)
Anyway, the formatting should be used consistently throughout the post.
It follows from this that if some expressions in the post have to be set using MathJax, then all other similar expressions should be set using MathJax as well.
$\displaystyle a_0 = \frac{4\pi\varepsilon_0\hbar^2}{m_\mathrm{e}e^2}$
where $e$ is the elementary charge
(not where e is the elementary charge)
$\mathrm{NH_4^+}$ is titrated with $\ce{NaOH}$
(not $\mathrm{NH_4^+}$ is titrated with NaOH)
Notwithstanding the above, we should not demand that everyone use MathJax.
If necessary, we may kindly ask for it.
MathJax supports many different commands; the syntax is confusing and its use is not intuitive. A suitable documentation is hard to find; our own explanations are scattered over several meta posts. Therefore, it would be disproportionate to require that all users learn how to use MathJax before they are allowed to ask a question. (Note that although $\TeX$ might be widely used in some disciplines of academia, it is rarely used in the real world outside of academia.)
Furthermore, as already discussed here, an introduction to MathJax does not solve the formatting problems. Even if new users know MathJax, they mostly do not really know what to do with it since they do not know how to properly write quantities, units, and equations. Preferably, any introduction should first state the requirements for quantities, units, and equations and then show how these requirements can be fulfilled with MathJax.
Anyway, users usually do not read introductions, guidelines, or help texts; they read existing questions and answers. Thus, new posts tend to adapt to the quality of existing posts. Therefore, we should be a good example to new users by writing properly typeset questions and answers as well as by making conforming edits including explanatory comments.