I'm in the process of replacing an existing Vent/Heat/Light unit in my
bathroom. The old wiring setup on this system seems odd to me.

The previous installer is running three Romex wires to the unit.
There is a two gang switch controlling (with one double switch).
One wire holds the Neutral and ground (other wire is clipped), coming
from source junction box.
Another wire is holding two feeds from the switch (the ground is
clipped).
The last wire is holding one feed from the switch (the other wires
are clipped).

Here is my question:
Due to new location of the new unit, and existing cable size, I need
to extend the two feed Romex cables coming from the switch. Hence, I
need a junction box to extend the lines. The issue is, if I use a
metal box then shouldn’t those wires be grounded to the box?
Normally, I would say yes ground the wires to the box. However, the
previous installer cut the grounds on those cables… so what value
would there be in grounding them? Another solution is to use a
plastic junction box (which is legal in my town), to avoid the
grounding issue.

Any thoughts?

Thanks,
Raj