In general it is possible to symlink wp_includes but not wp-admin. The main difference is that various places in admin want to know the path and they calculate it relative to the file which was first invoke to handle the admin request. The problem is that for symlinks PHP returns the path to the actual file and not its virtual path, making it impossible to find the resources (plugina, themes, whatever) of the specific wordpress install.
As implied from the first part, I did try to do it, but right now I don’t see the value of attempting that. With the improvements in PHP parsing you just get less value from improving memory/cpu by parsing one file instead of 5, and this setting goes against the philosophy of separating unrelated resources to help localize potential problem, as a failed upgrade in site A can just bring down all your sites.