music-functions: #$var1 or just $var1 ?

[Mark Polesky]

In both:
  Notation 5.6.1 "Substitution function syntax", and
  Extending 2.1.2 "Simple substitution functions",

it says that variables inside the music block part of a
music-function definition should be referenced using the
hash-cash notation (#$), but this clearly doesn't work in
several significant cases:

  \tempo \markup { \bold #$tempotext }
  #$note

In fact, I did an experiment where I took every example from
these sections, removing the # from every #$, and all the
examples still compiled fine.  Are there any cases where the
#$ is actually needed?  Also, for cases where both #$ and $
work, is there any reason that omitting the # would be bad
practice or something?  Any additional insights would be
appreciated too.

Thanks.
- Mark