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does show what can happen if the "wrong" people get setails of your guestbook!
Browsing a number of guestbooks, I could see that there was a problem on most with spam. That problem is presumably growing because of the current SEO focus on guestbooks.
If you promote it, then you will have to spend time each and every day clearing out the spam.
Given the cheapness of submission services, $100 will get you submitted to 1,000,000 guestbooks (I never knew there were that many), I think I would keep clear of promoting my guestbook at the moment ;)