Disc brakes are absolutely not necessary, just very nice to have. I have a commuter/secondary touring bike with canti brakes, and a tourer with discs which I use occasionally for commuting. Without a doubt, I prefer the discs, but having said that good quality v's or cantis setup well are more than adequate (I personally find replacing the pads with koolstop salmons is the best cheap brake upgrade you can do).
If I were to buy a commuting bike or touring bike again, I wouldn't have discs top of the list of priorities; having said that, there's no way I'd want to take them off. If money were no object, I'd get discs, but if I was on a limited budget I think cheaper good quality traditional brakes are more than adequate and I'd either not spend that extra money or spend it on other components to upgrade. In my opinion disc brakes offer low bang for the buck compared to their traditional counterparts, even though they probably offer the best performance, just at more of a premium price.