That's the sad thing: you're saying that it still turns a net profit after paying all of its staff and costs, and yet that's apparently not enough to keep it running.
Pro accounts could:
1) remove all advertising from the site. 2) have access to content early like drafts of upcoming articles 3) have access to revised or updates to articles 4) allow people to recommend articles
the list could go on, you'll probably think of a thousand things that I couldn't since I'm not in marketing.