It might be said that generally any user who's going to the effort and expense to restore and replinth a classic turntable (whether idler, direct or belt)-- is going to want to consider a twelve inch arm.
Same sort of thing, really. Like the table, this kind of arm isn't necessarily convenient, or plug-and-play simple, or stylishly 2012-looking ... but it is likely to be better, offering performance gains on what's currently available as 'stock' equipment.
From what I have owned and what I have read on the topic, any good-condition classic arm (Sme, Ortofon, Fidelity Research) and any currently available upmarket arm-- Ikeda, Sme, Orto, Moerch, Jelco-- is going to offer better performance than the nine-inch variation, assuming good setup. These companies are not in the business of offering questionable gear, so the standard "not as rigid" excuse just doesn't come into play with a perfectly functional unit. Even the other standard, "too much mass" isn't necessarily an issue, given that MC carts, as mentioned above, generally require increased mass.
So any of the top-shelf twelve-inch arms, if fully functional and well-matched to cartridge, is going to have the odds in its favor over the shorter ones. The end result of the twelve-inch edge on tracking is a more relaxed, fuller and more openly natural sound all the way through the Lp side, rather than any sense of confinement at the off-nulls, a feature of the shorter arm.
I'd offer that this is not one of those audiophile I-think-I-hear-it improvements. If you are thoroughly familiar with the performance of your Lp setup, you'll hear it clearly in one side of one record. This is also the kind of thing that is most noticeable when you have to go back to a shorter arm; there is always a more congested feel at the off-nulls with the short arm.
You might also notice as you look at the literature that the most immediate dismissal of the twelve-inch arm generally comes from owners or manufacturers of tables that could never accomodate a long arm anyway.
For my part, I regard the nine-inch arm as a concession to convenience over performance, something that a no-holds-barred project will not settle for. If you're already playing Lp Records, and on gear you've built or modified yourself, perhaps thru tubes, transformer-volume-control, Lcr-phono, and the kinds of esoteric speakers we tend to use (ie, not Polk or Bose)... you're not going to settle for the higher tracking penalties associated with short arms.