As far as Google is concerned, USB Mass Storage on Android is dead and buried. The last Nexus to have it was the Nexus 1; neither the Nexus S nor the Galaxy Nexus had it. It's useless to complain about it now.
The reason is that USB Mass Storage requires Android to unmount that storage, which plays havoc with apps trying to use that storage.
While MTP isn't a great solution (especially for Linux), QuickSSHd works for me, and adb is also an option.