Although not as important as single-blind study in my opinion, double-blind study does indeed present added assurance to the accuracy of the trial results at the end. Because the investigators who interact with the subjects will not be aware of who receives the real drug and who receives the placebo, there is absolutely no possibility for an investigator to become biased. For instance, an investigator may feel obligated to give a patient the real drug when he or she is given the placebo. Or perhaps the investigator, for whatever reason, does not want a particular subject to take the real drug and provides the placebo instead. As you mentioned however, it does add more time and even risk to determine which subjects had which treatment to obtain the results. There is added risk because there is a higher chance that certain subjects may have been believed to consume the drug when they actually had the placebo or vice versa. All in all, for trials that require extremely accurate results and trial is conducted properly, establishing a double-blind method would be beneficial.