The thing about vim is that it readily supports both modes of operation. I'll open one long running vim process per rails project I'm working on, and I use the --remote option on the command line if I want to open related files in that running vim.

But it's also nice to pop open a new vim in any old directory to deal with one or two edits that don't belong to a real project. I do this all the time when dealing with docker or Dev ops type tasks that don't require managing a lot of code.