By default, when connecting an Android device to an Ubuntu computer, gvfs is used via gphoto2 (because gvfs doesn’t have native MTP support yet). And that doesn’t always work properly.
Here’s my experience with this: trying to mount my Android 4.2.1 phone fails most of the time (I’ve experienced the same behaviour with Android 4.1 too), usually with some “Sorry, could not display all the contents of “Android”: Timeout was reached” errors. It does work at times, but it’s very slow and with such an unreliable behaviour, an alternative is always nice.