Grab's service is exceptional, and it only works in an app whose document format can contain graphics. It works with Create, TIFFany, TextEdit, etc. without needing anything selected. They just need an open document to paste the results. Of course, these apps work with services properly. It doesn't work in PS Elements 2, and it probably doesn't work in any Adobe app, nor any Microsoft, Macromedia or Quark app. That's the apps' fault.
But as a rule, services work on selections. They need some _thing_ to work on.
Remember, services are essentially advanced copy-paste actions. They copy the selection to the clipboard, paste it into another app (actually, a daemon I think) in the background, manipulate the copied data with that app's tools, copy the results back to the clipboard and paste them back in place of the selected item.