1. 

Q: Create another example, where nested trxns seem to be reasonable.

A: We can think of a holiday reservation system, where there are 
several servers involved. Each server runs a subtransaction of the 
trip-reservation-transaction. One subtransaction is responsible for 
reserving a flight to the desired destination, another of the hotel and 
third for desired day-trips at the destination. 

The coordinator initiates the appropriate subtransactions at each server. 
Each subtransaction can have multiple subtransactions of their own. There 
might be several flights and hotels to choose from. If at least one flight 
and hotel subtransaction succeeds (even if all the rest fail) the whole 
transaction can be committed. If the day-trip reservation transaction can 
not find any day-trips, it might not be that serious, we can still commit. 
If more than one flight or hotel is found, we can present different 
choices for the client.

Thus, nested transactions seem to be reasonable, because the success of 
the transaction does not depend on success of each of its part.