I always thought the advantage of programs like Screen and TMux were that you could persist remote sessions over bad connections, or from different terminals (e.g., going home from the office and keeping the session alive).
For purely local sessions, I don't see an advantage over iterm + vim. That also seems to be the conclusion on StackOverflow [0]:
[0] http://stackoverflow.com/questions/10015575/tmux-vs-iterm2-split-panes
[edit] - thanks for the feedback guys, I'll give it a try