Haven't used tmux much, and last time I used screen was over 10 years go. Rubymine gives me my iterative test results, multiple independent launch commands, the ability to run a single rspec test, refactoring support, autocompletion, a coherent gem environment, debugging etc all in one place. Yes, I could do all this on the cli, but I'm lazy, and Rubymine means I don't need to.