Are Social Bookmarking backlinks beneficial to a PR2 site ?

Absolutely. You want to keep building links over time as steady as possible. If you get enough of these links they can improve your pagerank although its not a guarantee. I use the tool below to schedule out social bookmarking, and have it all automated...its a nice tool.

And don't just do social bookmarking, also include article and video marketing as well as good old fashioned link exchanges with other sites in your niche. Diversity is the key.