Shareholder Rights Plans are one of the most common things around. They are there because management is in the best position to evaluate an offer (i.e. management knows of inside information that the outside investor doesn't, etc.). It does not take away management's fiduciary obligations to look at bonified offers.
Typically, a plan like this will result in a higher price being paid for a company in a take-over situation, because it allows management to bring the suitor to the table to negotiate (and base the negotiations on the inside information).
IMHO, This should not be viewed negatively, and is a positive in most situations. It just means that someone cannot manipulate the stock and swoop in and purchase the company at a less than optimum price.