Today I was trying to find a site (that I already knew, but didn't remember the URL), it wasn't in the bottom with the others (still in beta), and I spent several minutes trying to locate the list of all sites (that I also knew that existed, but forgot where it was). The path I end up taking was:
main page --> my profile --> my network profile --> all sites
From a new user perspective, wouldn't be useful if there were a "more", "all sites" or "beta" link in the bottom, for increased exposure? Just like putting area51 there (but even more so, since these sites are already active, but many people might be unaware of them).
P.S.: at the risk of looking stupid, I searched the main page source code to double-check the link wasn't really there before hitting "Post", and found out an alternative, single-step way of reaching that list:
main page --> "StackExchange" top-left menu --> all sites
That's better, but usability-wise I still think the bottom is the right place for it - that is where you're looking at when browsing the list of sites, and there's no hint there might be more active sites beyond the listed ones (unless you already knew about them).