Without this check the code continues to write into output file even if the device is hopelessly ENOSPC, potentially wasting lots time and silently corrupting the output.
The magic constant 1 check per 10 000 output packets causes 0.4 % overhead in my tests where whole input is cached and write goes to /dev/null.
This is not perfect - errors from the underlying close() operation are not reported. Unfortunately this is a limitation in pcap interface and we cannot do anything about it.