Some implementations set the ATT bit automatically and provide you a command or knob under a configuration stanza to disable it. i.e on Juniper: ignore-attached-bit
Ignore the attached bit on IS-IS Level 1 routers. Configuring this
statement enables the routing device to ignore the attached bit on
incoming Level 1 link-state PDUs. If the attached bit is ignored, no
default route, which points to the routing device which has set the
attached bit, is installed.
Others, require you to actually manually configure the ATT bit. i.e Cisco: attached-bit send
To configure an Intermediate System-to-Intermediate System (IS-IS)
instance with an attached bit in the Level 1 link-state packet (LSP),
use the attached-bit send command in address family configuration
So whichever it is, you certainly have power over what you want to achieve, in this case you have decided to perform leaking to have specific routes advertised to your internal area. In this example you have given, leaving the attached bit and thinking about a worst case scenario where there are no failsafe mechanisms, you could end up with a blackhole if you loose connectivity to the specific routes because you still have the default one.
On the other hand, I'm not sure which parameters have been taken into consideration for the leaking, so if it were to be something dynamic where the routes are only leaked if certain conditions you specified are met, then if one of those evaluates to false you will stop seeing those routes and this could potentially result in connectivity loss if there is no other way to work around, hence default route.
In regards to LDP not creating a FEC for the default route introduced by the attached bit in ISIS.
Personally I haven't tested this yet, as soon my hands get on some equipment will do, but for what Google has showed me:
/mpls ldp set distribute-for-default-route=yes does. It seems to add
an entry in the MPLS forwarding table for the default route
- MPLS switching does not have "matching longest prefix" feature of IP routing.