The suggestion is that using basename.tld instead of www.basenamne.tld adds to the security matters you need to think about, if not now then later if/when you add features on a subdomain that you (and/or your users) want to keep separate in terms of cookie sharing.

In that sense using www.basename.tld is thinking about (or at least autonomicly mitigating, by way of scope limiting) those potential security/privacy issues.