I work as a writer while in grad school (thesis-based Master's level though), and would say that getting experience writing about science for lay audiences is valuable because it opens up your career options and may improve your own grants and manuscripts, but I would not do it unpaid.

A few things to consider about the opportunity would be who will see the writing/how high profile is the publication (are we talking about Time vs your school's campus publication)? Who will you be networking with by doing the job? Do you get to interview the PIs who you are writing about, or do you just read the papers and write about them? Does any of this fit into your career plan?

Also - are you sure it would be that intense/time consuming? I would say it takes me a half of a day to write a column of 600 words or so, and I generally am interviewing the PI so I spend an hour or two preparing questions, 15-20 minutes conducting the interview, and 3-4 hours writing and editing.