I think having a LinkedIn makes all the sense in the world. Besides the obvious benefits, you can ask for recommendations directly on LinkedIn, so if admissions officers scan your profile, they can see recommendations from your previous experiences. In particular, this idea makes a lot of sense for those with part-time jobs and leadership experiences.
My personal experience last year: for schools that let you upload a resume, I remember doing that, and on my resume, I have a QR code to my LinkedIn.