Checkboard
Almost exactly this scenario. But not just subsequent TR unit failures. Any short circuit in any device connected to a bus will pull the voltage on that bus low and disrupt the operation of other unfaulted loads until its (the faulted load's) circuit breaker pops.
With the DC tie closed, both DC buses are operating as a single bus, susceptible to any single fault. While cruising, it's not a big problem to wait a few seconds for a breaker to open and other connected devices to reboot and recover. Not. something you want happening while landing. Particularly when you really need that ILS working (low visibility, etc.)