If you have 6 app servers it's just daft to stick 6 NICs in your DB
server.
While there might be some cases where that makes sense most likely it
isn't something you would want to do. I believe the original motivation
was to solve bandwidth congestion rather than security.
If the NICs are saturated then channel bonding is the answer. Any modern switch and OS will let you bind together multiple NICs so they appear as one. You can get an 4Gbps link this way.
