If I'm not mistaken, the UE1 Pro is a one-side professional IEM. You might notice that some performers will wear one of the two pieces, as they like or need to hear more of the stage and crowd sounds/noise than wearing both will allow.
The UE1 Pro is for those types of performers, as it's one side only, and, instead of sacrificing one of the channels for the one-side wear, it combines both channels into that one piece.
It's a specialty item for the pros, and would be of little interest to most Head-Fi'ers (for obvious reasons).