If the website you are visiting also integrates +1, then there is no need to go back and +1.
But yeah, going back to the Google search results page is a pain and I doubt very many people would actually take that action, unless they are really impressed by the site and are familiar with the +1 feature.
Google's usage comes from leaving Google's site as fast as possible (search for a term, then leave Google to visit the result).
This looks like you would do a search, visit the resulting site, and then remember to go back to the Google results page to click the +1.