You should feel free to continue to write portable scripts.
That is horrible advice for anyone who is just starting out though and they should stick with #!/bin/bash and not worry about it until they actually want to consider platforms other than Linux. Portability is the wrong thing for people who are learning to focus on.
It is also bad advice for most system administrators at most businesses since they won't have those alternative *nixes either, and its a best practice to minimize how many different operating systems you have to care about.