proxy_set_header inheritance not working?

[Ed W]

Maxim Dounin wrote:
> Hello!
>
> On Fri, Apr 11, 2008 at 01:48:12PM +0100, Ed W wrote:
>
>> Hi
>>
>>> Quoting has nothing to do with variable expansion.  Variable 
>>> expansion happens unconditionally if it's supported for particular 
>>> parameter.  This is usually documented on per-directive basis.
>>>
>>> Please note: quoting supported for directive arguments, but not for 
>>> complex entities within them.  E.g. the following won't work:
>>
>> Is it possible to request variable expansion for log file location? 
>> (ie easily make it a function of server name).
>
> No.
>

...Because...? 

Is it a bad idea?  Hard to implement?  Something else?

Ed W