if HADOOP_IDENT_STRING isn't specified explicitly, it defaults to $USER.
pid files that incorporate HADOOP_IDENT_STRING will then have different names when invoked by different users, negating their effect as uniquely identifying a process using that conf dir.
slaves bind to the configured port, but if that fails they scan forward up to 10 ports to find a port to bind, so they'll start just fine, connect the the same master and presto, we have two slaves on the same machine running as different users.
I always use the --config parameter, so yes, I'm not overly concerned, but I've seen this happen.
It's not a big deal, but as long as you're going to the trouble of hashing the conf dir into the file names, I thought it might be another improvement.