Say you find 20 PR6+ sites and do this every day for just one week. Then you could end up with 140 PR6+ high quality backlinks. How many sites in a small niche can compete with that?
Remember that you just have to do it once and then reuse all your findings on your new sites.
Thinking about it this would actually be one hell of an e-book on how to dominate a small niche.
If you want to save yourself some time, it might be a good idea to add "comment" as a footprint when searching for backlinks (comment link:domain.com/url/) to filter out all pages without comments. Will save you a lot of time if one of the sites build backlinks with forum profiles.