It's very unusual not to include the piano part in a vocal score. The only
reason I can think of is that the piano is doubling the choir and not
playing anything of it's own. If the piano is adding something different,
then it should be included in spite of the space it takes up. There are
several reasons for this.

1. Vocal music is normally sold on mass for the entire choir, choral
conductor, and rehearsal pianist. They will all be using the same score and
not separate parts.
2. The choir often need to see what the piano is doing so that they can get
notes and spot important cues.
3. The individual singers will want the piano part if they take the piece to
a teacher or just want to practice at home and play through on the keyboard.

I've only ever come across a couple of vocal scores that used multi-bar
rests and they caused much more trouble than they're worth. To put it
bluntly, singers like counting even less than percussionists. Even
Beethoven's choral fantasy, which has about 10 minutes worth of instrumental
introduction, included a piano reduction of everything up to the first
choral entry.