Why not have the machines presented to the clients or other interfaces by a virtual IP from an application delivery controller such as F5's BigIP or similar? An

Why not have the machines presented to the clients or other interfaces by a virtual IP from an application delivery controller such as F5's BigIP or similar? And then remove the dependency on static NIC's on a virtual appliance? Seems counter intuitive to run virtual appliances on static addresses if it can be avoided.