Usually the cable guide (the piece to the right of the clamp) is only necessary on bar and stem shifters because it is not inline with the bike downtube.

. Why do you think one was put on the Unishift downtube shifters? To save money by using the piece from the stem/thumb shifter?

The guide provides stops for the lever travel and also keeps the cable from slipping out of the groove. It should always be there afaik