Most balancers have a 0.02V tolerance. A bit more than this is fine under most circumstances, but can be dangerous in the wrong hands.
For instance, a 2 cell pack that balances to within 0.05V: While charging you might see 4.15V, 4.20V at full. Not a problem if your charger reads individual cell voltage, but for a non-balance type charger, all it knows is that the voltage is 8.35V and it will continue charging both cells. Overcharging is very dangerous and I think balancers should always be used.
When discharging, ESCs dont have individual cell checkers, so its a similar situation to a charger without balancer. If you are set to a 3.0V/cell cutoff, or 6V cutoff for a 2 cell, and then one cell is at 3.05V, this means the cutoff will not kick in until the other cell is at 2.95V. Not usually a fire hazard, but not good for the cells at all.