All else being equal, a 12" inch arm should be better than a 9" arm (who can argue with more inches is better, guys?). The trouble is, there is no way to make all else equal. It is hardly a technological breakthrough to make an arm longer, but to do so, one must increase mass, particularly inertial or effective mass because the heavy cartridge will be hanging way out there on the end of a lever. Either that or a lot of mass has to be removed from elsewhere which compromises rigidity.
It is much easier today to make the compromise in favor of longer arms than in the past because of the available material. One can use carbon fiber impregnated material, cast magnesium, titanium, etc. But, it is not necessarily the case that any given arm can be scaled up to a longer length with ease; some designs might suffer too much in the other areas of concern, such as rigidity.
So, it is a matter of designers making their choice of tradeoffs. But, to say that one aspect of design, specifically length, is so important that a long arm is inherently superior, as someone mentioned that Roy Gregory has suggested, I don't buy. I will take a "short" Triplanar, Vector, etc., any day over the VPI arm.
