first benefit is i can use a second monitor/other porgrams without having to alt tab very useful for streaming. (or the second monitor.)
Exactly. I absolutely have having to wait a few seconds for the game to get out of full screen before I can actually navigate to another screen or window.
By having it full screen + windowed mode, I can easily just move my mouse over to another window without having to wait.
I switch between windows quite often, so you can only imagine how annoying this would get. About to try Steven's suggestion right now.