Check the file uploaded, it is PHP 5.3.x after all, so extensions mostly should work.
And about the usefulness of extensions: Who uses all of them any way?! I am open-source CMS developer, and I am not even utilizing half of those extensions that come packaged with official release. I firmly believe it's only a matter of perspective.
EDIT 1: And yes, you are right about including different architectures of PHP... It will take a lot longer to test them, and by the time you release it, PHP will have newer version
Additionally, that's true - PHP x64 most probably won't work with Apache x86 as a module [and maybe also in CLI/CGI mode, but it's only maybe, have to make sure], so it''s not worth it...
EDIT 2: Oh btw, by Memcached extension, I meant php-memcache, not php-memcached. Although php-memcached is much superior to the other one, it's not supported in Windows environment, since it specifically utilizes libmemcached component of *nix systems. That's why you will have to put php-memcache in Windows packaging. Together with Membase server community edition, they can do a miracle - one step closer to Linux environment development. And of course, you can include BOTH memcached extensions in Linux packaging of XAMPP. They are very easy to find, as you know
Man, I love Linux...