It causes a lot of issues/limitations, which are costly to fix/mitigate;
The value this feature brings to the application is dismissable;
Changing/managing voices is still difficult, despite all time and effort invested here;
Presence of voices complicates the (already complicated) syntactic Tree (without voice two, we could move Clusters directly under Measures, and remove the floating selection toolbar);
Voices 2 is always played back as Piano, regardless of the Part it is in, which is a limitation of the underlying ABC2MIDI.EXE that we are using. Fixing it implies a lot of work and (further) limits the number of staves one can use in the score.
On the same page, the ability to decide on how many staves a Part should be notated is a before-its-time-feature that should be removed, at least until the application gets more stable.