Drone strikes in Pakistan the "least bad option"

Peter Bergen and Katherine Tiedemann have written an informative analysis of US drone missile strikes in Pakistan, in part prompted by claims that a disproportionate number of civilians have been killed by Predator missiles.
While some have claimed that up to 700 civilians have been killed in the drone strikes analysed by the authors, they suggest the number of civilian deaths since 2006 lies somewhere between 250-320 - around 32 per cent of all casualties.
They say the attacks have killed around 530 people since President Obama has been in office, including five or six senior leaders of jihadi groups. Of those, between 250-400 are reported to have been lower-level militants, with around a quarter being civilians.
President Obama has dramatically increased the number of drones strikes; 43 this year so far, compared with 34 in all of 2008.
Bergen and Tiedemann point out that although the drone attacks can be judged to be successful, they may be on shaky legal ground. They quote Columbia Law School professor Matthew Waxman: "The principle of proportionality says that a military target may not be attacked if doing so is likely to cause incidental civilian casualties or damage that would be excessive in relation to the expected military advantage of the attack.... But there is no consensus on how to calculate these values (how do you compare the value of civilian lives versus the value of disrupting high-level terrorist operational planning?) Nor is there consensus on what imbalance is ‘excessive.' It's very hard to draw definitive conclusions because it requires assessments about such things as the expected military gain from neutralizing the target, the likely civilian harm, and the availability of alternative means of attacking that could save innocent lives."
Bergen and Tiedemann's judgement?: "Drone strikes will remain an important tool to disrupt al Qaeda and Taliban operations and to kill the leaders of these organizations, but they also consistently kill Pakistani civilians, angering the population and prompting violent acts of revenge from the Pakistani Taliban.
"For the time being, however, they appear to be the least bad option the United States has for reducing the threat from Pakistan's militants, given that an American ground assault into Pakistan's tribal regions is out of the question, and that U.S. and Pakistani strategic interests are more closely aligned today than they have been in years because of the two countries' shared interest in attacking the Pakistani Taliban and their al Qaeda allies."