Situation at moment of USB plugging to PC it's not so important. I am using USB BSL a lot, and if every time for flashing USB was plugged/unplugged, port and/or MSP430 will be dead long time ago.
For entering in USB BSL programming mode, there is no need to disconnect/connect USB cable from/to PC with BSL button pressed every time. USB cable can stay connected, and another entry to USB BSL mode can be done using RESET button. Just press BSL and RESET button together, first release RESET button, and after (1 second is enough) release BSL button.