Why the sudden popularity of 12 inch arms

Tracking error issues/differences are nothing compared to the destructive effect of crappy bearings. Great bearings is what put SME on the map after all!

Beyond great bearings, straight line tracking is the best since it mimics the action of the cutting lathe; but its benefits can only be realized by solving (at great expense) the other mechanical problems straight line mechanisms present.

To me, a 12" arm that is both lightweight and free of resonances implies a cost/benefit ratio that is not going to be favorable compared with a 9" arm.
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Much earlier in this thread, I said the same thing Raul just stated -- namely that the tracking error reduction from a 9 to a 12 inch arm will be sonically insignificant compared to other factors. I think it's also important to remember that the original reason for having 12" arms was for playing broadcast 'transcriptions' which if I remember correctly were 16" in diameter -- it had nothing to do with reducing tracking error.