Is Google+ actually useful for SEO?

Yes it is one of the factors which the quality team at Google adopting

You can find more information on this link: http://seo-hacker.com/google-affect-seo/.

Hi Roba, Google+ is the future of SEO (in a way).
Google has been fighting spam for years now and whenever there's an update, spammers find a way out to cheat the algorithm.
Google has been trying hard to identify the source of content available online through Google+.
So consider Google+ as watch dog for Google.
Webmasters are linking their sites with their official Google+ pages and are also implementing Google+ authorship tags.
This helps Google identify the source of content and also helps understand the authority of a person or a company online.
Spammers can't have their spam profiles on Google+.
So ultimately, everything available online will be divided into two categories.

1- Content identifiable by social identities like Google+, Facebook and Twitter linking.

2- Annonymus content.
People also like content from people they know and they'll prefer reading content from known authorities instead of unknown experts.
This is creating an echo system where the authoritative content will rank higher and authority can only be achieved by genuinity and quality.
So in short, Google+ is playing an important role in SEO and it's importance will increase with the passage of time.
This is trus for other social identities as well but Google+ has an edge as Google has access to database of all Google+ users, Google+ posts are indexed in search engines however, Facebook locks it's content (except public posts) and Twitter can't only help in number of retweets of that 140 characters.
Hope it helps.
--- Thanks Usman Latif Founder, Internet Marketing Pakistan™ http://about.me/usmanlatif