Why? I mean, if you've already gone to the trouble and it works for you, then great. But as a solution for the rest of us?
3mm neoprene booties are good down to the teens, °F. Below that, slip chemical hand warmers on top of the toe of your shoe and under the bootie, that's worked for me down to single digits °F. If that didn't do the trick, I'd probably switch to Sorrel boots with flats. Or, more likely, ride the trainer or go skiing or snowshoeing. (If it's that cold then there's likely snow on the ground, why not do some cross training?)