Google+ has suffered some setbacks lately. Granted. For example, 90% of those with Google+ accounts are not active. And I can see why. The benefits of Google+ seems to be less effective as a social media venue overall. Some swear by Google+. Some swear by LinkedIn. Some swear by Facebook and others swear by Twitter. Each has it's place and each suits a purpose better than the rest. As for Google+, coercing usage was not a good idea. You have to wonder why Google would do this. As it turns out, there is a good answer for this despite the low usage, the removal of some features, some features becoming products of their own, high employee turnover including key personnel, lack of focus, etc.
Google does look to social signals, however, often, the reality does not match the hype. For example, much of Facebook is not available to search users without an account or with access to private profiles where a connection is required. I imagine that LinkedIn can be much of the same. As for Twitter, while it is more open, it suffers from short posts that are short on relevancy. Twitter now allows posts and that seems to be a clue. This means, as I search for something, not being a Facebook user, many of the Facebook results found in the SERPs end up being useless. In fact, even with a public profile, Facebook answers few questions. It is not where serious people go first to post serious information.
Another aspect is that while social media has it's place, it seems to be best suited for trends. And this is exactly where Google might spike a sites web page presence. While Facebook posts may not rank well and links seem to have little value, mentions of a web page suddenly gone viral, even for a short period of time, will temporarily place a page much higher in the SERPs.
All well and good. But how does Google+ benefit a site?
While some, including myself, will argue that Google+ posts has little effect, one thing is clear. Google+ is more open. Posts are indexed more readily, often do not require an account, and links from these posts are said to have an effect. That said, I rather suspect whether a Google+ profile is active and focused or not makes a difference. Recently, I was poking around some Google+ accounts for the most popular SEO sites. Some were focused on the topic of the website, and others meandered around much like a personal profile. Same with Twitter. I did not look at Facebook.
So let's split things into several camps.
For businesses, especially for local search, Google+ for Business seems to have a real effect. To begin, a card in the knowledge graph is very possible if some simple branding signals are created on the web site. It is possible these days that smaller businesses will see their business pop up in the knowledge graph. It is also well understood that for local search, having a Google+ for Business profile is rather helpful.
For blogs, especially one that is prominent or where authority can be established, Google+ has the ability to vet the author. While the author tag is dead, what was not said at the time was that any other attempts to determine authorship died with it. I believe these efforts to be as fully alive as they ever were. Tying a Google+ profile to a site and author is a step in the right direction as far as vetting the author and knowing exactly who that person is.
For personal sites, I am not sure there is enough benefit that can be brought except within narrow margins. I suppose it really depends upon the person, the site, and the effort.
Keep in mind that establishing authorship authority on the web is nearly impossible. In the early days, Google used, and likely still does, it's ability to parse simple data such as names, addresses, phone numbers, etc. into a semantic link web that could establish some authors fairly clearly. Patterns would arise that would make authorship and authority fairly clear for some. With that, mapping social media profiles added more valuable clues. However, there was still a lot of noise in the data. Noise reduction within the data set left most authors out of the loop. Afterall, is this Steve Jones the one in South Bend or Roanoke? For example, a true story. I was a consultant within the IT industry using my personal name as it was incorporated. This worked well until just about half a block away someone with my same name, in the same field, specializing in some of the same things began to consult using his name. It caused some confusion between our customers. I sought him out. Hell of a nice guy! We had to come to an agreement. My name is not so common but not as rare as hens teeth. Until recently, there was only one of us in a whole city. If it was confusing for us in a small market, how is it for Google with a global market who knows little about anyone?
Another consideration to keep in mind. Google does nothing for the heck of it. Google does everything to collect data they hope will be valuable for search. For example, GMail is primarily a link discovery tool and was created with this in mind. With Google+, the idea was to establish trusted author profiles that can be vetted and trusted. Hence why Google coerced people to join even if they do not participate. Google has vetted profiles that could be useful. While LinkedIn, Facebook, and Twitter account profiles belong to others, vetting these profiles externally was and likely is a semantic link map nightmare. For Google+, it is direct. Google knows the author because Google holds the account and can vet the author as much as they want. And they do. Ever get a letter from Google or a phone call? Likely the answer is Yes(!) to both questions.
So in that respect...
Where Google+ still has an advantage is in Googles ability to trust data. There is a silent push for trusted data over content lately if you haven't noticed. In the knowledge graph, all semantic data links much be corroborated. For featured snippets and rich snippets, much of the same rules apply, though less strictly. You can see how Google's SERPs can be influenced by pushing data through old forms, parsing, or new forms, schema.org markup, and by using Google+ profiles. Did you get that? Google is relying more upon data that is passed in trusted ways these days to influence the SERPs. Exactly where and how this is done, would require a longer answer.
So is Google+ worth the effort?
For a business? Yes. Abso-freakin-lutely!
For an authoritative blog? Depending upon the authority and popularity of the blog and whether Google+ is used actively and in a focused way. But yes. Very likely.
For a personal site? Perhaps. But mostly socially I suspect - specifically if you work it. Any established authority may be less serious.