Its Google's product
I guess this reason is good enough to prove that it does help in SEO, but there is one reason which proves that it does help in SEO, it is Google places which has got integrated with G+ and now it is called Google plus local, This is an important factor for business listing.
i Would like to say ,Google+ Very useful In Digital Marketing .it's one type of Social Media marketing . If you use Google+ Authorship it's Also very useful For Seo. i m used google+ Authorship and i got Very nice Result .
I am seeing a very positive affect of Google+ on SEO because it is helping them a lot to generate a good traffic, also as a Google's product it is really helpful in increasing PR of a website as it get indexed easily.