Having the server use SSI causes a slow-down on the system.

Think about it. Instead of just grabbing a file and spitting it out to the calling client, the server has to open the file, look on every line of the file for SSI instructions, do the SSI instruction, assemble the document, then spit it out to the calling client.

This can be costly in server performance. It can also be a great boon to website developers. Image having one menu file that is stuffed into each html page during the call of the page. That way if you change the menu all pages receive the updated menu.