Advertisers are rightly embracing "viewability" as a way to ensure their online ads' efficacy -- but brands shouldn't be too quick to adopt the metric as the key currency for their marketing efforts, writes Ari Paparo. There are plenty of kinks still to be ironed out, including the metric's applicability to video and native ads, its vulnerability to bot fraud, and its lack of international acceptance. "Moving ahead with a currency too quickly will be a huge setback to display and will reinforce the perception that it is hard to measure and subject to technology for its own sake," Paparo warns.