There is another use case where fractional extra note spacing would provide better appearance for layered staves. When whole notes are on adjacent staff positions, the "weak side" note is moved by more than one note width, so notes on a layered staff that have extra note spacing of 1 will not quite line up. See the following clip.
!NoteWorthyComposer(2.75)
|AddStaff|Name:"Staff"|Group:"Standard"
|StaffProperties|EndingBar:Section Close|Visible:Y|BoundaryTop:12|BoundaryBottom:12|Lines:5|WithNextStaff:Layer|Color:Default
|StaffProperties|Muted:N|Volume:127|StereoPan:64|Device:0|Channel:1
|StaffInstrument|Trans:0|DynVel:10,30,45,60,75,92,108,127
|Clef|Type:Treble
|TimeSig|Signature:4/4
|Chord|Dur:Whole|Pos:-3,-2
|AddStaff|Name:"Staff-1"|Group:"Standard"
|StaffProperties|EndingBar:Section Close|Visible:Y|BoundaryTop:10|BoundaryBottom:10|Lines:5|Color:Default
|StaffProperties|Muted:N|Volume:127|StereoPan:64|Device:0|Channel:2
|StaffInstrument|Trans:0|DynVel:10,30,45,60,75,92,108,127
|Clef|Type:Treble
|TimeSig|Signature:4/4
|Chord|Dur:Whole|Pos:-7,-5|Opts:XNoteSpace=1
!NoteWorthyComposer-End