SHIELD would be a serious deterrent for trolls who have their eye on large companies with the means to defend themselves. But trolls eat startups first and a startup is often unable to fight through the courts and get to the point where SHIELD would help. If the troll is after something like $1,000 from every company using a scanner then not many businesses are going to risk going to court. And if the troll isn't interested in any reasonable settlement then the legal fees and management distraction can kill you.
SHIELD is well intentioned and would certainly help. But we need to stop examining patents before issuing them and do the job properly for the few that ever get used in anger.