Seamless integration with parts that don't need to be GC-free comes to mind: they are not building an application, they are building a building block. And that building block can be used both in applications that do require the latency guarantees of GC-free as well as in applications that don't. Another class of applications would be ones that alternate between phases of unpredictable latency (like bootup or reconfiguration) and low-latency operation.