Having been in the industry over 8 years, I'm almost embarrassed to ask, but it's something that been bugging me:
Considering that pressure/energy loss is almost always undesirable, and the fact that they're not easily pigged, why and under what circumstances would a reduced port ball valve ever be a better choice than a full port valve?

Full port cost more, and lots of applications don't need them. Installations like domestic water service (especially commercial & residential), many compressed air lines, isolation valves for steam traps, etc.

Reduced port ball valve is the normal choice because of its less cost as compared to full port ball valves.When less pressure drop is desired (but not very critical) then I have seen reduced port ball valves. I have seen the full port valves normally in inlet of PSVs( here the minimum the loss the better).Ball valves can be used for control purpose as the flow and pressure drop can be set in the range from full to completely closed. Is this possible with
venturi.And ball valves are not only for control thay are used as block valves also (very often in gas service).

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