why not: because if the i/o fails, your script will run forever, using 100% cpu, doing nothing.
Mh, yeah, I had a similar problem with my scripts. No infinite loop, but a blocking write to a fifo, as my fifo listener process was killed off. I wasted lots of memory with my stale php processes. Sorry about that! I fixed it by adding checks for the fifo listener (using pid files) and aborting the php script if the write would be blocking.