Okay, here's my usage. (Everyone else's will vary :) I have six spaces (i.e. screens), and I often have windows open on all of 'em.

The first one is for Mail; I have the Inbox window, any emails I've yet to read or reply to or do something with (always a number of these); and any other windows I open while writing mail.

The second is my main web browsing window, with MacReporter and Safari. (At least one Safari window with some tabs, coz there are always a few pages I've not got around to reading; I may open another window with more tabs for specific purposes such as researching and buying products or specific web searches.)

Third is for iTunes. Along with the main iTunes window, I'll also have other windows open temporarily, such as Finder or Safari or MP3Trimmer when I'm adding new music.

Fourth is for TV, with the main EyeTV window, and also its viewer window (when not full-screen). I'll also open Safari etc. windows if I want to look up something related to what I'm watching.

The other two windows are used as and when, but it's rare for both to be empty. For example, when I'm working on some software, I'll have Eclipse or Script Editor or Terminal open for editing, plus Terminal and/or Finder windows for testing and that sort of thing. Or I may have several Safari windows if researching something, to keep them separate from my general browsing. Or I may be writing some music in Cubase or Lilypond, both of which need several windows. Or using Amadeus Pro to edit programmes recorded off the radio. Or, well, anything that's a concentrated task and needs most of the screen.

For example, I've just checked, and I currently have 26 windows open across 5 screens, with the other one empty.

Does that give you any ideas?