Got to jump in here real quick. Coaster brakes can be very useful depending on the type of commute you have. What the OP describes could be a route suitable for coaster brakes. There are disadvantages to coaster brakes, of course. Reliability, however, is not one of them. I'd say coaster brakes are vastly more reliable than cantilever brakes. And for every day use a coaster brake will outperform a rear wheel canti anytime.
Obviously a secondary brake system is always a very good idea.
However, I've used coaster brakes for little while now, and have to say I haven't even touched my front cantis yet.