Kurt,
You are correct that a cat with a traveler does not need a boom vang. I think one reason the earlier Hobies (14, 16 and 18) were set up for boom vangs was because so many of them were sold into rental service, and rental places wanted to keep things simple for novice cat sailors and also not have to mess with maintaining a traveler system. So lots of rental boats did not have travelers at all and used the boom vang instead. That's just my theory, but it makes sense.