> * I feel like I'm getting something for nothing*

You pay in copper loss, because you are using only 3/4 of the copper (assuming primary and secondary have equal amounts of copper).

Or: primary resistance loss is double what it could be if you could use the entire available space for useful copper.

In signal transformers, lightly loaded, this is not a big deal.

In power transformers (including audio when you need every last watt possible), it means poorer regulation and more heat.

A 600CT:600CT transformer will typically have 30 ohms resistance per 600 ohm winding. Using half the winding, you get nominal 150 ohms impedance with 15 ohms resistance. Or seen end-to-end: as 600:600 you have a total 60 ohms copper loss, 10% or 1dB; but as 600:150 you have 22.5 ohms copper loss against 150 ohms impedance, a 15% or 1.4dB loss. However mike inputs are not usually power-matched. 2,000 ohms is a typical input impedance. The 22.5 ohms of copper loss (instead of the possible 15 ohms) is only 0.1dB loss, negligible.

> *a cathode follower mic*

In that case, the 2K mike-amp input impedance reflects back into the cathode as 8K, a nice high value for good output and low distortion. But it is still a 600 ohm winding. Assume they mean "600 ohms at 20Hz", and assume your cathode impedance is 2K. Ignoring small factors, your -3dB point shifts up to 67Hz. Actually, many 600:600 trannies do better than 20Hz, and if you run the tube rich it may be less than 2K cathode impedance, so it may not be that bad. If you care, disconnect the grid from the capsule and drive a signal generator through a cap to the grid, see what comes out of the transformer. Shoot for signal levels of 10mV and 100mV, because inductance varies with signal level.