The shape of the port, whether round or rectangular, is important from an understanding of what shape is best to efficiently manufacture plug valves. The issue of effective seat design plays a huge role in determining the shape of the port. In smaller size valves - up to 12" - the most cost effective approach to produce plug valves is with round ports.
As you get into larger sizes, the most cost effective approach to producing plug valves is with rectangular ports. However, this comes with a caveat. The more divergent from center the flow must take in a rectangular port, the greater the amount of head loss the valve generates.
To measure or compare one valve manufacturer to another in terms of head loss, the plug manufacturer who has the larger port width, on the surface, will have less head loss than a manufacturer with a narrower port width.