i think the impact of instant previews on traffic is being diffused. here are two reasons i think this:
mobile apps - they have opened a new way to generate traffic without requiring users to pass through the serps. the percentage of mobile users varies for each site. my network averages about 10% of traffic from mobile users and this number grows each month.
stupid users - google does a great job adding new features but lets not underestimate how stupid users can be. a large number of people still go to google.com and search for "google". i bet many users dont even know to click on the magnify glass icon to see the instant preview.
that being said i do think we should figure out how best to address instant preview. some options to think about is:
should you cloak a pretty page for googlebot?
should you fill out short pages so they dont look empty?
should you increase the use of h1 tags, to make previews easier to read?
should you shrink font size & make users click through to read?
those are some of the ideas that are bouncing around in my head about instant preview.