Using a different font here makes it stand out in a nice way.  But note
different x heights that makes the line spacing uneven and rather ugly,
so it's not ideal here, either.

>> (defface fixed-pitch
>>   '((t :family "Monospace" :spacing mono))
>>   "The basic fixed-pitch face."
>>   :group 'basic-faces)
>
> "No effect" in the sense that if the default face uses a proportional
> font, fixed-pitch defined as above still uses a proportional font?

It makes no difference either way, whether `default' uses a fixed-width
font or not -- the `fixed-pitch' font uses "Monospace", which is a
fixed-width font.

> I actually meant something like this:
>
>   (defface fixed-pitch
>     `((t :font ,(font-spec :spacing 'M)))
>     "The basic fixed-pitch face."
>     :group 'basic-faces)

That leads to:

Don't know how to purify: #<font-spec nil nil nil nil nil nil nil nil nil nil 100 nil nil>