Everyone focuses on why learning metric would be better in the first place, and they're right. No one has come up with a good argument for me to throw away all of my measuring tools, convert all my recipes and relearn an entirely new system when the system and stuff I have works for me now.
Because otherwise it will be difficult to live in a country that has converted to metric, presumably. While I'm not pushing for us to change to metric (even though I know it would be good), I suspect the change would be fairly easy to adjust to.