When I use the cycle charging dispatch strategy in my simulations, what happens when I don't specify a setpoint SOC for my battery bank?

The battery Setpoint State of Charge (SOC) has no effect on whether HOMER charges the battery.  The setpoint SOC only affects how long HOMER continues to charge the battery.

The definition of the cycle charging strategy is that whenever the generators run, they run at full blast and charge the battery with any excess power.  That’s true regardless of the presence of, or value of, the setpoint SOC.  But in the absence of a setpoint, HOMER will stop charging the battery whenever it feels like it.  That might be after only one timestep.  So, without a setpoint SOC, the system sometimes leaves the battery lolling around at low state of charge for a long time without charging it up.  That doesn’t bother HOMER at all, but it could kill lead-acid batteries in real life.  The setpoint says that when the system starts charging the battery, it will continue doing so until the battery reaches the setpoint state of charge.