I would say it is an absolute necessity. Charge controllers have a tendency to fail resulting in a dead short (connecting + to - on the battery, resulting in a fire, explosion or something in between).
I had this happen on a properly wired off-grid system (bad diode) and it would have been quite bad had it not had a breaker of the appropriate size. I would not sleep next to a battery that wasn't fused or breakered.
Reminder: DC systems and AC systems require differently rated components. You cannot just go buy a 30A breaker from home depot for the DC side of your system. It takes a lot more to disconnect DC than it does in AC (because AC cycles to 0V 60x/s it doesn't take much of a gap/insulator to break a connection, DC is not the same. Trying to use a light-switch for a DC light can result in problems [1].
I've had good luck with midnite products (charge controllers and breakers, etc) [2]
