lucy24 - 1:59 pm on Feb 13, 2013 (gmt 0)

In addition to slowing down the server, it makes a mess of your logs. They'll change from this

67.122.aaa.bbb - - [12/Feb/2013:19:14:33 -0800] "GET / HTTP/1.1" 403 799 "-" "<here I am>"

to this

adsl-67-122-aaa-bbb.dsl.pltn13.pacbell.net - - [12/Feb/2013:19:15:08 -0800] "GET / HTTP/1.1" 200 682 "-" "<here I am>"

(I've seen this before but had to double-check. In this particular case the IP address is still extractable, but in some log entries it wouldn't be visible at all. In others it may come out backward. Plays havoc with log processing, anyway.)