The problem with a suit like this is that it costs the plaintiff nothing, because the Yale Law School clinic is paying the bill, and would cost a typical defendant (Yoo says that he has friends who will defend him for free) lots to defend. Even if the clinic loses, and badly, it gets publicity for the charges leveled at the defendant, free p.r. for the clinic, a fundraising schtick, and an intimidation tactic to use against future government officials who disagree with its policy positions. Even if the typical defendant wins, he gets stuck with a legal bill, hundreds of hours of wasted time and energy, and a stain on his reputation from charges that are effectively exempt from libel law because they are part of a legal complaint.
In other words, it well pays the Yale clinic and likeminded activists to file lawsuits like this even if they are pretty sure that they have no chance of winning. It makes one wish for loser-pays, or at least the old version of Rule 11.