I own four Austrian 'shelter quarters' (i.e. zeltbahns) which make a great tent for the kids. When I first got it, I - being too curious for my own good - fiddled with the panels in all sorts of configurations, tent-wise and raingear-wise. Besides confirming to my wife that I am quite mad, I concluded that the zeltbahn is superior to the poncho in all respects save one - a critical one: You can't sleep in it. Folded in half, a 'zelt makes a cone shape which is no good as a sleeping bag cover/bivy. When I used my poncho out in the field, typically it was as a ground sheet, bivy, or tarp - things a single panel 'zelt does poorly.
Hope this helps.
.....CLIFF