What's the benefit of balanced tonearm cables?

I can only answer your question in theory, since it would take actual listening to tell if there is an audible difference in your system between a balanced and unbalanced cable.

Balanced cables are normally recommended for situations where you need to carry a weak signal over a substantial distance. Here are two examples where a balanced cable is probably desirable:
1. carrying a relatively low-voltage audio signal from a microphone to a recording device that is 30' or more away;
2. sending a signal from a preamp to a power amp that is 15' or more from the preamp.

If the distance from your tonearm to your phono stage / preamp input is less than 5-6 feet, it is unlikely that you really NEED a balanced cable. Infact, some audiophiles (including members of this forum) state they prefer the audio quality of unbalanced RCA cables to balanced XLR cables. To determine for yourself, you will have to do some extended comparisons.

A phono cartridge is by nature of the design a balanced source to begin with. If you're lucky enough to have a phono stage with balanced inputs then by all means do take advantage! The benefit over unbalanced is reduction in hum & noise pickup via significantly better common mode rejection ratio.

Or pseudo balanced rca as apparently my arm cables are. I have no idea what anyone is saying! I asked a techie and he said a cartridge can only be left and right so not balanced and the arm guy responded and said my arms are pseudo balanced, so pretty well nearly fully balanced. As I have no way of comparing or altering it is yet another one of those wierd techie issues that drive us normal people to distraction if we stop long enough to wonder what they are all saying.

The reason you go with balanced line is to eliminate any cable artifact, and by that I mean that you may have noticed that some cables sound better than others. That goes away with a properly set up balanced line system.

Balanced line and single-ended (RCA connections) are inherently incompatible. If you have one then its not the other. There is no such thing as pseudo balanced- that would simply be single-ended.

Now all phono cartridges happen to be balanced sources. Anyone saying otherwise is simply misinformed. The coils of the cartridge will work fine if they are hooked up backwards- all that happens is they are out of phase. If you did that with a single-ended source like a tuner you would get a huge buzz.

You know that little ground wire connection that most tone arms have? The one that other single-ended sources don't seem to need? That is there there to deal with the fact that a balanced source has a ground connection that is independent of the signal. Without it the balanced source, run as a single-ended source, will buzz. Its easy to hook up a cartridge in the balanced mode- in most cases you don't do anything with the tone arm wiring. Its usually about the interconnect cable that goes between the arm and the preamp. To do this, the preamp really does in fact have to have a balanced input.

The advantage of doing this is that the interconnect cable at the source of the stereo will not have any effect on the sound of the system. If you used a single-ended setup, the cable would have an effect and would have to be chosen with some care.

I've never heard of blanced cabled cancelling cable artifacts. What they DO measurably do is reject common mode noise.  EMI/RFI is often like that.  It induces a current in the same direction in both conductors which cancel each other out.

They may also have increased capacitance or inductance as a result of the construction.

For a tonearm, it's very interesting, since most cartridges are inherently balanced, using a balanced input stage could lead to less noise pick up, and better detail retreival.  COULD. No idea if true.