[BrianG]

Low resistances can also be measured with a conductance meter (reciprocal of resistance). However, measuring low resistances like that is hard because the meter leads and connections can affect the measurement appreciably. For this reason, measuring voltage drop is easier. Why would you need a lab quality PS though? As long as you have an accurate and calibrated voltmeter (with the proper resolution), you could measure the voltage output of any PS and the V drop on a small resistance (shunt maybe).

Also, IIRC, the internal resistance of batteries is not linear. Instead of supplying the resistance value of cells, manufacturers should supply the resistance curve.