KiwiC,
All live effects apply only to how the previewed art is generated.
None of them cause changes to the selectable art or generate additional selectable art.
To get access to the modified art, you need to do an Expand Appearance. If you do an Expand Appearance on your path that you have applied a live Outline Stroke command to, you will see that the expanded appearance contains a filled shape instead of a stroked shape.
Why would someone want to outline a stroke as a live effect when a preview of an outlined stroke should look the same as a preview of that same stroke before outlining? (Discounting deficiencies in Outline Stroke such as its inability to handle dashes.)
The answer is that all by itself, it is not much use. But when combined with other effects, it is.
For example, make an object with a very thick stroke, like 30 pts wide. Duplicate that object. To one of the copies, apply an Outline Stroke effect. Now to both of them, apply a Zig-Zag effect. Move the Zig-Zag effect down to below the Outline Stroke effect. (By default, Zig-Zag is applied above the paint layers.) On the stroke that has the Outline effect, the zigzags will be applied to each edge of the stroke separately. On the one where it doesn't, the zigzags will be applied to the centerline path.
Here are some interesting effects you can achieve by compounding this notion:
The Outline Stroke effect can also be useful in conjunction with the Pathfinder effects. Kurt Gold has posted some cool examples of those.
Here are a couple of links:
vice86, "Can strokes be gradated?" #, 30 Oct 2005 7:30 am
Scott Falkner, "Effect to release compound" #, 17 Aug 2005 11:23 am
(Unfortunately Scott seems to have removed his images that he linked to, but Kurt's are still working.)
See also the file "Outline Effects demo.ai" in this folder:
ftp://tpettit.best.vwh.net/AI_10_Style_demos.zip
As far as preserving a copy of your unoutlined stroke, the easiest way is probably just to copy-paste in front, and then outline the top copy.