This is an interesting point. The www. prefix seems to be slowly fading away from the landscape, but URLs without the prefix never felt like first-class citizens on the web.
The inability to use CNAMEs in root level domains also affects other aspects, such as load balancing [1].
My reading of the heroku post seemed to indicate that the recommendation is for heroku customers only. Of course, if your own systems are using reverse proxy to serve up content, then the advice may also apply to you.
I think "www" will make a comeback with increasing popularity of non .com/.net/.org TLDs. "whatever.io" looks like a web address to most of us here, but not to many others, even computer-savvy "normals".
The inability to use CNAMEs in root level domains also affects other aspects, such as load balancing [1].
[1] - http://blog.y3xz.com/post/3920967238/the-anomaly-of-amazon-e...