Well - depends on how serious you are, and what the climate is.... I've been out quite a bit in the Winter time here in MN - photo is of us out in February in the Twin Cities.... normally riding with a full dry suit with long underwear underneath, full dry gloves, a hoodie... and I normally ditched the booties as they don't really fit in the bindings....
It's fun, but honestly - not fun enough to go through all the work of getting dressed for it....
Quick tip - We would normally drain the hot water from our water heaters at home and bring it with us in a cooler, so once you were done riding... you could warm up your frozen toes/fingers in the hot hot water....
I think people do it now, just to say they have done it - honestly - I would rather be on the slopes as well...