I like coaster brakes on city bikes and I am very comfortable using them. My preferred transportation bike set-up is to have a coaster brake in the rear and a hand-operated hub or rim brake in the front. Typically I use just the coaster brake most of the time, employing the hand-operated front brake to keep the bike still when coming to a complete stop or to supplement the coaster when braking at high speeds. The role my front brake plays in these scenarios is small, but crucial - which I realise more than ever when riding the coaster brake-only Sogreni I picked up last week.
Finally, there is the question of speed. When going over 13mph or so, I find that I cannot brake as well as I'd like with just the coaster brake alone. The coaster brake is enough to slow down, but for an emergency stop at that speed I need a front brake to supplement.
Riding a coaster brake-only bike is certainly possible, but in a city like Boston I feel that it is safer to also have a front hand-operated brake. The motivation behind bikes without front brakes today seems to be the "clean handlebars" look, which has always baffled me. I fail to see what is so gorgeous about not having a brake lever on my handlebars, when that brake performs an obvious and necessary function.