What's the benefit of balanced tonearm cables?

This might be a little off topic.
I'd like to make a statement, and Ralph is probably the only one that can comment on it.

Let's look at the waveform coming from the cartridge. Because it is a balanced source, it looks like a pure Sine Wave. We can all envision this, because we've all seen it.

Pin 2 of the XLR carries the positive (rising) portion of the sine wave, while pin 3 carries the negative (falling) portion of the sine wave. A dual differential circuit amplifies both halves of the waveform. Eventually the positive portion pushes your driver 'out', while the negative portion pulls your driver 'in'. We get pure sound.

In an RCA connection, the center pin carries the positive portion of the waveform. Your driver moves out. But what happens to the negative portion. This is now referenced to ground. Ground can be envisioned as the center of the waveform, meaning the driver is only pulled halfway down. For all I can tell, the rest of the negative portion of the waveform is 'thrown away'. My question is: what is pulling the driver all the way back in?.

If you have a dual differential amplification chain, stay with XLR. The entire waveform will be preserved, at least until it reaches your 'unbalanced' parallel speaker crossover. 

I can clearly hear the differences in balanced cable....are you saying that balanced cables sound alike?

They will if set up correctly. You might want to take a look at the Audio Engineering Society file 48 which defines the balanced line standard. I the shield gets driven by the source you will experience cable artifact- IOW various cables will sound different, although not nearly as much as with single-ended setups.

But eliminate cable effects? That’s a bit of a stretch for me. It does not cancel out capacitance, inductance or resistance.

Its not a matter of cancellation- its a matter of reduced IMD and swamped cable artifact due to low impedance operation. In the case of a phono cartridge, the source impedance is quite low, maybe only a few ohms, often terminated at the other end by a low impedance as well. Its hard for a cable to express any artifact of capacitance or inductance in such circumstances.

your bat gear is a balanced design and would benefit from balanced cables. Many companies simply provide the option for balanced or single- ended at the rear to accommodate customers (pseudo or fake). generally a balanced design will sound better running balanced cables-my gear has always been single-ended and sounds better that way. many of my friends talk about the increase in volume they get with balanced but do not focus of the sound quality difference if any. it was originally designed for longer runs in studios etc.

From the phono amp designers I have spoken to over the years it's not a simple case of one design being better than the other, but a question of implementation.
I recall one designer saying that he gave a balanced option to satisfy the demands of the high end buyers i nthe states and far east. He said that introducing the extra circuitry for the grounding in a balanced set up caused of itself problems, and wrongly done can cause sound.
Two of the greatest stages of all time were un-balanced - Vendetta and Mares/Connoisseur.

Balanced signals have greatly reduced common mode noise,

Just to be precise, balanced signals can have common mode noise, maybe even a lot. It's that balanced circuits reject the common mode noise and only amplify the differential signal.