No, most of the time they are not indexed by search engines - unless the docs has got a lot of good backlinks, it will not be visible through search. Also, a lot of docs which seem to do well in Google search are scam pages with illegal movie downloads, etc.

So there is a lot of added value in showcasing and promoting great docs which are not easy to find. Another area is giving feedback to the doc owner though commenting - the collaboration system in Google docs is not ideal for high-level discussion about the doc.