The main use of multicast address for neighbor discovery is used to reduce the Processing overload of routers.If the packet is broadcast any Layer 3 device has to process atleast upto layer 3.If it is a routing protocol,it has to see the destination port whether it is opened for that protocol or not.The best example is RIPv1,which uses broadcast for sending update or to tell its presence.

If eigrp is enabled in the interface,it belongs to the member of 224.0.0.10 multicast group.Any frame received with multcast address 01:00:5e:00:00:0a will be processed completely.If 224.0.0.9 at layer 3 and 01:00:5e:00:00:09 at layer 2 is received at eigrp enabled interface,it simply going to drop the frame without processing it.

As far as I know multicast address in routing protocols like EIGRP,OSPF,RIPv2 reduces processing overload of routers