I would say the vast majority of boats are rigged with both a vang and a traveller. The vang is not good for upwind applications because of the much higher leech tension that is required. You would need a very powerful purchase system and/or a winch to get the kind of leech tension that even a modest mainsheet on a cabintop traveller system could provide with only arm strength. The other shortcoming, which you mentioned, is a biggie; every time you adjust your vang you have to readjust your mainsheet. That alone would justify having a traveller!I think it was me who said we had a boat with a single mainsheet and traveller - but I don't know what a German style is.
I don't understand why you say the boom can't be centered and twist controlled. The twist is controlled by the vang, and the mainsheet can bring the boom to center. You need to readjust the mainsheet after letting out the vang, but the boom does come to center.
The shortcoming is that the boom cannot be positioned to windward of centerline. This is helpful in certain conditions. However, the vastly superior control of a vang compared to a traveller or twin sheet setup when deep off the wind more than makes up for it in a cruising situation.
Best of all worlds would be a traveller or twin sheet with a vang. More complicated, though.
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