I doubt you should EVER connect it when the power is on but regardless, that would not change durability from metal fatigue. Connectors of that type or intended for occasional use, for example linking boards together in an assembly that may never require disconnection or only for rare maintenance purposes. It isn't intended for regular plugging and the figure of 300 will be before it is no longer guaranteed to meet specification, it would probably survive in most applications many more times.
Brian.