merge summaries

It took me some time to figure out how to get summaries of the merged branch into merge messages. Finally I recognized that I simply need to set

	git-config merge.summary true

I have two questions:
1) Why isn't 'true' the default setting? The two probably most  
prominent git repositories, git and kernel, both use merge summaries.  
After some time working with git I started to wonder, how these guys  
create the nicely formatted branch summaries. If merge summaries are  
apparently best practice, why are they missing in my local merges?

2) Why does git-merge have a switch '--no-summary' to switch  
summaries off, but doesn't have a switch '--summary' to switch them  
on? After some time of searching for the solution I started to  
believe that Linus and Junio might use some shell magic to create  
their merge messages, which they pipe to git-merge's -m switch. This  
inconsistency made finding the simple solution even harder.

- Steffen