I generally get the idea of "multiple voices" in music such as explained in this question and answer.
Is it also correct that in modern (e.g. non-classical) music multiple voices are used to show different instruments in a more compact way?
For example, in this part of "With or Without You" it looks like the bassline is in the bass clef and accompaniment chords are shown with stems pointing down and the vocal line is shown with stems pointing up.
Also, I am sure the best answer is, "whatever sounds better" but is there a convention as to what a Pianist is supposed to play when there are multiple voices?
For another example, this section of "Hang on Sloopy" (found here) is confusing:
Although there is an extra treble clef dedicated to the sung part of the song, it seems like the same notes are added to the accompaniment part of the score. Additionally (having tried it) playing only the stem down notes from the lower treble clef makes it sound like things are missing (for instance the first F note), however it is clearly in a different voice.
Even if someone could play all of the right hand notes, later sections of the song are quite awakward to play all of the notes in the middle trebble clef.
Is there some sort of standard to know what is expected to be played on Piano when looking at sheet music like this?