For a PhD pursuing a career in academia is a Linkedin helpful?
In my experience (academia and industry), there is very little benefit to LinkedIn in the academic sphere. It just fails to have critical mass in academia, and therefore gets used very little. I'm sure it depends on discipline, but a combination of own/departmental webpage, preprint servers and publication aggregators, and (in some disciplines) Twitter is where it's at. That being said, a low-key LinkedIn profile doesn't hurt, especially if you want to start staying in contact with friends and (former) colleagues who leave academia and for whom LinkedIn becomes a lazy directory of who has gone where and how to stay in touch. I can't imagine it would have a negative effect on perception.
I do however recommend to academics (including advanced Ph.D. candidates) to set up their own, personal (i.e. professionally focused but personally owned, controlled, and managed) webpages, especially if they are in techy- or in any way outreach-to-the-public relevant fields. These days, it is cheap and easy to get a domain name of your own, to set up a WordPress (or similar) site somewhere. Then you get to post your research in the format you want, show a bit of creativity, etc. And by doing it early and well, your site will rank highly in search. It's much more professional looking, to current- and would-be peers as well as to journalists and the public (if relevant) if you have a well-organized and up-to-date website, than if a couple of years down the road a Google Search for you yields 3-4 abandoned pages at institutions you did a post-doc at, with expired links to old papers and no mention of the most exciting topic you are focusing on at the present time. In this day and age you will eventually want to control how you market yourself, and it's easiest to do if you start doing it gently early rather than in a big rush when you discover you ought to have been doing it already.