Imagine a few of the following scenarios:
1) You wok for an ISP and have access through them. One large
enough that they apply their AUP to their own people. You have ISDN/DSL
or some other connection w/ reverse-dns for your personal domain @ home.
Someone drives by your place, finds your unprotected lan, sends spam, hacks,
etc.. complaints come in, you lose job because you were a spammer and
your employer needs to stop, etc.
2) You are a small company, someone does this, and you get
blacklisted as a spamhaus. you are unable to get internet access.
3) you have a cable modem as your only high-speed connectivity.
you have one of the linksys/whatever nat+802.11a/b boxen. you
get used, you get blacklisted and can not get high-speed pr0n again.
While these seem like minor annoyances in some cases, they
can be quite dramatic to the person on the receiving end. I wish
the wireless vendors would use a somewhat more inteligent approach and
turn WEP on by default when shipping their units and at the cost of
a few cents more they can print a sticker on the box that can be
removed later that has the uniqe WEP key for that unit. Similar to
the way when you go to the hardware store you can play match-up to get
the same key for multiple locks.