What are the downsides of using only one connector of a Y-shaped sub cable at each end?

[aspen63]

Apart from the price of course.

 

[Kal Rubinson]

Make sure the unused leg of the Y is shielded to avoid contacting anything or anyone.

 

[Zek]

What is the scenario behind all this - what exactly is being connected to what and which cable is in question?

 

[Speedskater]

Is it a mono cable with a wye at each end?
Or a stereo cable that wye's out at each end?