Why would you want that, and what do you want to accomplish with that? This can already be achieved by a simple Perl CGI script, PHP, or -oh, let's throw this back in- Server Side Includes, if I understand your point (what I probably don't do)...
If you take a typical web-shop page, such as say http://thinkgeek.com the whole page is standard issue - the same for every customer. All, that is, for the little bit in the top right which says "Loot: Your cart is empty.".
It would be great to save the whole page statically to the server and then only have to generate that tiny little bit from the CGI. This would allow some really heavy processing to be applied to each page, such as category generation and related products, safe in the knowledge that it need only be done when te database changes, not for each request.
It also would allow the page to be cached by proxies and would allow you to slurp small amounts of data from other sites - stuff that is currently done using images - such as hit counters.
Just because you have not felt the need for it does not mean that it is not desirable...
UPDATE: This could be made into a mod_perl filter is suppose - that is a thought...
--tidiness is the memory loss of environmental mnemonics
The decision of when to use SSI, and when to have your page entirely generated by some program, is usually a matter of how much of the page is static, and how much needs to be recalculated every time the page is served. SSI is a great way to add small pieces of information, such as the current time. But if a majority of your page is being generated at the time that it is served, you need to look for some other solution.
"Just because you have not felt the need for it does not mean that it is not desirable..."
Oh most certainly not :) If the case was wheter I felt a need for something to allow something's existence, the world'd be pretty boring :)