The scary part of a shareholder rights plan is the potential dilutional effect (eg how it works) to other investors. Most takeovers offer a premium over the current value to shareholders; so not all shareholders would want a "poison pill". Not only that, they do not always work if the intention is to avoid a takeover.

CFO magazine stated the following concerning this topic (link below):

"Since 1997, for every company with a poison pill that successfully rebuffed an unwanted advance and remained independent, 20 pill-protected companies accepted takeover offers."

http://www.cfo.com/article.cfm/30013...