I use a 2 wheel burley because that was the one that I could get second hand at the time.
I like it for touring on pavement because, as a clyde, it takes weight off my wheels.

I'm planning some off road touring this summer, have purchased a bike with 26" wheels and will get panniers for it.

I haven't tried transporting the burley by bus, train or plane. All I've done are "hub and spoke" tours from home.

The drag isn't a problem until the hills get steep... >8%