If you point a static route to a broadcast interface, the route is inserted into the routing table only when the broadcast interface is up. This configuration is not recommended because when the next hop of a static route points to an interface, the router considers each of the hosts within the range of the route to be directly connected through that interface. For example,
ip route 0.0.0.0 0.0.0.0 Ethernet0.
With this type of configuration, a router performs Address Resolution Protocol (ARP) on the Ethernet for every destination the router finds through the default route because the router considers all of these destinations as directly connected to Ethernet 0.
This kind of default route, especially if it is used by a lot of packets to many different destination subnets, can cause high processor utilization and a very large ARP cache (along with attendant memory allocation failures).
Specifying a numerical next hop on a directly connected interface prevents the router from performing ARP or each destination address. However, if the interface with the next hop goes down and the numerical next hop is reachable through a recursive route, you should specify both the next hop IP address and the interface through which the next hop should be found. For example,
ip route 0.0.0.0 0.0.0.0 Serial 3/3 192.168.20.1.
Excerpt from Specifying a Next Hop IP Address for Static Routes at cisco.com