RESET pin on target device is used for JTAG / SBW / BSL entry sequence, so FET (or any other flashing/debugging tool) can't do anything on target device if there is no connection to target device RESET pin.
- - - Updated - - -
To be complete clear, if target device have SBW (and MSP430G2553 have it) than target device RESET pin must be connected (even if 4-wire JTAG is used, and SBW is not used).
https://processors.wiki.ti.com/index.php/JTAG_(MSP430)
**broken link removed**