There is absolutely no evidence that having a profile and the authorship tag will impact your keyword ranking. Having your photograph in the rich snippet that appears against your search result does help improve your CTR.
I think that having an author profile is useful for the reason GifAnimator mentioned.
The thing you have to watch, though, is what the search results point to. We've found that if we post a teaser about content on Google + and include a link to our website, what shows up when you search for a closely related keyword is the link to the post you made on Google+, not a link to your own website.
Since you will always lose some of the traffic when a link points to one page, and then you have to click on something else to read more, I can't help wondering if there are times when Google+ posting isn't advantageous for getting visitors to a website.
That said, I guess it depends where you want to direct traffic. I've heard of people building followings on Google+ that bring in business, so you need to consider what you want to accomplish with posting, and then measure and test results to see what works.
At the end of the day, unless your exclusive business is making money with AdSense (which is risky at best as a business model), what matters is what brings in business.