it's exactly what you said. it's safer to use the 3 prong, but yes, it is also okay to use the 2 prong. the ground plug helps guard against electrocuting you. yes, the chances are miniscule, but I figure I get a longer cord AND I'm mitigating that minimal risk
on a different note, I've noticed my power adapter gets hot when I'm pushing the CPU, such as using Handbrake or gaming in Windows