This is because all the state is shared within a single Vim: buffers, copy-paste information, and so on.
I don't use tabs because I don't find them useful at all. If I need to see 2 files at once, split panes it is; if I am switching between files, I have them open as buffers(NERDTree and BufExplorer makes it pleasant).
> I use tmux as my window manager for my terminal, so I have not use for panes in vim.
I use tmux as well. The only times I use tmux panes is when I need to run shell in the same window.
> I just have a bunch of vim processes running and use tmux for the window management.
And I have one vim process running for one project root. Coding rails? Open vim in top folder and open all files from there using rails.vim navigation commands(:Rcontroller, :Rmodel...) or NERDTree and switch between them using BufExplorer.