role of the mirror branch

[Yoshihara Tadafumi]

Hello,

I started to use Bazaar in my team and I encountered 
the question, "why do we need the mirror branch?"

In 6.2 of the user guide recommends to create a local
mirror branch first and create task branches from it.
But I think you can create task branches from the trunk
branch on the server. And if you merge it with the server
trunk branch, the workflow would be equivalent with the one
described in 6.2 of the user guide.
The work flow would be like this:

 % bzr branch xxx/ABC/trunk   ABC-bug123
 % cd BAC-bug123
   (work, work, work)
 % bzr merge 
 % bzr commit
 % bzr push xxx/ABC/trunk

First, I thought that you can save the resource used by 
the shared repository with the mirror branch style. But as far
as the total file size of ".bzr", the above "direct task branch"
style consumes the same amount disk size. 

Is there any other reasons to create a mirror branch on the local
machine?

Thanks,
Tada