Our LHBS uses a copper manifold in their direct fire 3-tier MLT....you need to be very careful with the heat and constantly stir. They also fly sparge with that setup...seems to work out OK, it's the rig they use for all their classes.
Something is always fermenting....
"It's Bahl Hornin'"
Kegged: Bourbon Barrel Imperial Stout
On Deck: German Lager