Using the support library is encouraged in general since it contains the latest piece of code of every API. What B.Young is pointing on RenderScript applies to all the other APIs.
-------- No need to keep reading.
Let's put an example. Let say that when Android 8 is released they introduce a new class to hold values referenced to keys called FastMap. To add support for older versions, Google releases a support library with this new class, and two months later during performance tests they find a way to make it more efficient, so they release an update of that support library with the optimized version of FastMap (since Android OS is frozen into the phone).
Additionally, and since big number releases are always a bit rushed, Google realizes that they introduced a very specific bug (yehh, it happens), which gets fixed on a newer update.
So if were to build an Android app for Android 8 and above and used the built-in APIs, you'd be missing all these improvements coming in the support code.