\new Voice no longer automatic?

[Graham Percival]

I think I missed something... my understanding was that contexts were 
created automatically as needed.  In other words,
{ c'4 }

automatically expanded to
\score{
  \new Staff {
    \new Voice {
      c'4
    }
  }
}

However, in bug 176 (below), a progerror went away by replacing a \new 
Staff with \new Voice.  What's happening?

\new Staff {
  \override Score.SpacingSpanner #'strict-note-spacing = ##t
  \afterGrace c'4 {c'32 c'32 }
  c'4
}


Cheers,
- Graham