MSP430G2553: measure the voltage of a non-rechargeable 3.6V battery
I see two advantages to Circuit 2, but neither are critical here. As with many design questions it ultimately comes down to weighing benefits against the cost, both of which happen to be minor in this case.


When PIN_CONTROL is inactive, the ADC pin sees a high impedance in Circuit 2, whereas in Circuit 1 the ADC pin sees an undivided 3.6 V. If your ADC were actually running at 2.5V that might cause issues, but in this case it's just a 2.5Vref and the MSP430 can accept VCC + 0.3V on any of its input pins. So we don't really care about that here.
If you were concerned about leakage current at the ADC input, Circuit 2 would prevent it from continuously drawing from the battery. I don't think the MSP430 ADC is well-characterized but I did find this forum post on the TI forum that quotes it at 50 nA static leakage current:


Input impedance of ADC in MSP430F5438