why is a 13" tonearm design really superior?

HAHAHAHAHAHA, LOL. Yeah there are trade-offs when the arm length becomes to large. The change in effective weight placed on the stylus proportional to changes in up and down vertical movement increase with arm mass. So yeah don't plan on using a flag pole as a tone arm just yet. Nonetheless, a slightly longer tonearm certainly does decrease tangency error for the reasons given above, it is simple geometry. The optimization problem is slightly more complicated as Baerwald was helped by 12 guys from nasa, as I understand it. Well, I have not found it to be too difficult, nothing a little calculus of variations and free parameter sensitivity analysis of some parametrically defined functions can't solve. IE> a slight bit beyond highschool calculus if you want to do it from scratch, and 9th grade math if you would like to use Baerwald's, Loengren's, or Stevenson's optimal null points tweaked for your turntable.

Flyfish - agree with you on your Phantom judgement. I did run the Phantom I (with some upgradings) against the "longer" SAEC 8000 on my Nakamichi - means on the same TT, same Crystal Dreamline cable to the KSL transformer, same Titan I cartridge. In my eyes the Phantom is one of the best tonearms you can buy at the moment - and it fits on most TTs. I like this tonearm very much. Nevertheless against the SAEC 8000 the matchpoint is on the SAECs side.
The problem of the SAEC 8000 is the headshell, which needs to be a very special one due to geometrial reasons. And they are hard to find. Phantom tubes are usually available.

One thing you all forgot, the 8000 has 1.25* tracking error at the lead in of a 12" LP which gets less & less as the arm goes toward the center of the record. At the lead out the tracking error is 0!!! No other arm on this planet does this!!!

Dear Antslappy, that was a common concept of SAEC with their top-of-the-line tonearms.
The SAEC 506/30 is very similar.
However - you can get that kind of alignment with most every 12" tonearm which allows for some alternation of offset angle and overhang.
It is not an exclusive built-in-feature of the two big SAEC tonearms only.
It is a matter of concept and whether you wish to align that way.
Cheers,
D.

One designer(who makes 12 inch arms) told me that they didn't hear any sonic improvements above a length of 12 inches.