The way mmap()'d IO is handled in cassandra is dangerous. It allocates potentially massive buffers without any care for bounding the total size of the program's buffers. As the node's dataset grows, this will lead to swapping and instability.
This is a dangerous and wrong default for a couple of reasons.
1) People are likely to test cassandra with the default settings. This issue is insidious because it only appears when you have sufficient data in a certain node, there is absolutely no way to control it, and it doesn't at all respect the memory limits that you give to the JVM.
That can all be ascertained by reading the code, and people should certainly do their homework, but nevertheless, cassandra should ship with sane defaults that don't break down when you cross some magic unknown threshold.
2) It's deceptive. Unless you are extremely careful with capacity planning, you will get bit by this. Most people won't really be able to use this in production, so why get them excited about performance that they can't actually have?