I would like to be able to have multiple instances of Workbench open in my Mac OS. I could use that in order to refer to other workspaces while I'm creating a new one. I often copy and paste parameters and transformers from another FME workspace. It's more difficult to do that when I can only have one window open at a time. I'm also just looking at the flow in different workspaces from different projects. I'm not always plagiarizing myself from just one workspace.
I also tend to work on more than one thing at a time. Because people in the office will ask me to change something in another workspace that I made while I'm creating a new one.
I don't know of any way to open multiple FME Workbench windows on OS X now.