Rear brakes get useful in sloppy conditions where your front wheel is likely to lock up and skid rather than you endo'ing. Also good in cases where you have a lot of extra weight on the bike where using only the front would likely result in failure of the fork. (surly big dummy readme)
Both so very true. Rear brakes are a cornerstone in winter, IMO. Additionally, I wouldn't be taking my Xtracycle down any hills without them..