> Perhaps users constantly coming back and checking their comments for replies, increases time on site, and the possibility users will interact further and view other posts.

Joel Spolsky wrote a piece on Building Communities with Software back in 2003.[1] He reasons that allowing email notifications is "the best way to kill dead any young forum" and that such "checking back" as you describe helps a forum to achieve critical mass and survive.

[1]: http://www.joelonsoftware.com/articles/BuildingCommunitieswi...