My commuter/errand bike is 48/18 in the summer and 48/20 in the winter, fixed. Ice and snow is only a consideration a handful of days out of the year for me and rain is rain. I gear down because the wind tends to pick up in the winter. It also seems to help keep me warmer when I'm spinning faster, but that's just as likely a psychological effect.

All in all, I don't really think it's necessary unless you're going to be slogging through snow. Keep your current setup and save a bit of money.