They are not required, but you get better performance by using the native maps. You can build them by going to $ACCUMULO_HOME/src/server/src/main/c++/ and run make.
I am using Accumulo 1.4 rc5 which I checked out and compiled. After installing on our cloud, the tserver logs were showing an error loading the native libraries, I do not recall which now. However when I checked my accumulo version it did not even have an lib/native directory.
I was able to find and use the native dir under the Binary distribution of 1.3.5 but now I am wondering how should I have acquired the native files? Did I miss a step when compiling?