[pro] why :key arguments?

[Tamas Papp]

On Mon, 04 Jul 2011 11:39:39 +0200, Hans Hübner wrote:

> On Mon, Jul 4, 2011 at 11:31 AM, Tamas Papp
> <tkpapp at gmail.com> wrote:
>> Why do some CL library functions have :key arguments?
> [...]
>> but it is a bit cumbersome.  I can make my code simpler by relying on
>> calls like
>>
>> (quantiles (map 'vector key vector) quantiles)
> 
> This not only conses "a bit more", it also duplicates traversal efforts
> - The original list must be traversed, and the consed-up list of key
> values as well.  I think it is prudent that the CL library functions
> offer ways to reduce consing for cases where "a bit" is too much (and "a
> bit" can become a lot if a program operates on long lists).

I understand this.  My main question is: why not do this with compiler 
macros?  Is there any reason for this, other than historical?

Note that I am not complaining about the standard, I just want to learn 
the reason for this design choice so that I can take it into account when 
writing my own libraries.

Best,

Tamas