In cases of fraud, deception, etc. suing is of course the right thing to do. But in this case the board of Yahoo has chosen to reject the offer from Microsoft based on their belief that the bid was too low. Where exactly is the wrongdoing in that? You can of course disagree, in which case you should take it up on the next shareholders meeting, and vote for removal of the board.
But I have a hard time seeing the justification in suing a board for having an opinion on the valuation of the company. I thought that was what they were paid to do...