I read about this a while ago -
Petr Lapukhov wrote -
By default, IOS code does not generate an IGP label for the default route. This allows for "fallback" routing of non-tagged packets, since every router may lookup the routing table for untagged packets. In some case this obviously may harden troubleshooting Of course, you can force LDP to advertise tags for the default route as well using the command "mpls ip default-route". (Note this only works for IGP routes and LDP generate labels). Tagged default route instructs PEs (ip2mpls border) to assign a tag to all packets not having a specific destination in the PE's routing table. This will essentially result in "tunneling" of these packets to a special "sinkhole" destination, originating the default route.