One activity where 'one thing at a time' doesn't work: research/invention. Edison was the classic multi-project guy: many research projects, most of them at any one time stuck waiting for him to find some way to progress them.
Serious researchers are forced to work like that. Some of them manage to apply it to the rest of their lives too. Don't ask me how, though :-(
You can learn to manage multiple projects after you master managing them one at a time. This doesn't mean you should be working on multiple projects at the same time though, multi tasking seems to be pretty solidly proven as less effective than serious focus.
Serious researchers are forced to work like that. Some of them manage to apply it to the rest of their lives too. Don't ask me how, though :-(