Second, the mirror requires a small voltage drop to maintain operation (to keep the output transistors of the mirror in active mode). As a result, the current mirror does limit the allowable output voltage swing, but this limitation is much less than for a resistor, and also does not depend upon the choice of bias current, leaving more flexibility than a resistor in designing the circuit.
The mirror needs a small voltage drop to function (to keep its output transistors active). This limits the output voltage swing, but less so than a resistor, and is independent of the bias current choice, offering greater circuit design freedom than a resistor.