[10char]

A constant bottom bar would take up screen real-estate reserved for the feed, which is where most of the user's time will be spent (mostly consume vs create content)

And even if there was a bottom bar, Path offers 6 different posting options. Putting all of them on one bar, or hiding them somehow with a slider, would make it seem awfully crowded.

[sandofsky]

More importantly, the bottom bar is for switching tabs, just like tabs in a browser. It doesn't make sense for performing actions.

A tab bar would be a worthwhile replacement for the slide out left menu, which adds friction to switching sections, and lacks discoverability. If you want people ever visiting those sections, it's worth the screen real estate you lose.