Null routes: Float or sink?

I've seen a configuration on customer networks that I just don't understand.

Let's say that some routing tier has been allocated the 10.10.0.0/16 network block. Because only some of that space is actually in use, a null route has been added so that traffic for unused portions of the space doesn't wander the network aimlessly:

Dist-A#show ip route 10.10.0.0 255.255.0.0
Routing entry for 10.10.0.0/16
Known via "static", distance 1, metric 0 (connected)
Redistributing via ospf 1
Advertised by ospf 1 subnets
Routing Descriptor Blocks:
* directly connected, via Null0
Route metric is 0, traffic share count is 1

Dist-A#


So far so good, but look what's on Dist-B:


Dist-B#show ip route 10.10.0.0 255.255.0.0
Routing entry for 10.10.0.0/16
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 1
Last update from 10.10.0.0 on FastEthernet0/0, 00:24:56 ago
Routing Descriptor Blocks:
* 10.10.0.0, from 10.10.0.0, 00:24:56 ago, via FastEthernet0/0
Route metric is 20, traffic share count is 1

Dist-B#


Uh-oh. Dist-B isn't trashing traffic for these unused destinations. It's learned the route from Dist-A via an IGP. Maybe they forgot to null route that block on Dist-B?


Dist-B#show run | include ip route
ip route 10.10.0.0 255.255.0.0 Null0 254
Dist-B#


Okay, I get it. Dist-A null-routes the unused portions of 10.10.0.0/16. If Dist-A fails, then Dist-B's floating (AD of 254) route gets installed so that Dist-B can null-route the unused space.

I'm sure it works fine, but I don't understand why the floatiness is useful. Someone had to design this and type it in. I would like to understand the motivation behind it. Why not just add a normal (AD of 1) null route to both distribution routers?

Is there an advantage to doing things this way? What is it?