I have been around horses, big and small, for nearly 50 years, and until AMHA brought it up, had NEVER heard of any specific, 'official' spot that was called the 'base of the withers' (BOTW). To me, BOTW has always been a general area, not a readily identifiable, pin-pointable spot. So, I am adamently oppose using this 'arbitrary' location as a new point of measurement. No one at AMHA could anatomically describe the exact point that designated the 'base'. I am still waiting for someone to tell me which vertebrae consitutes the 'base' ...
I whole-heartedly want to change to measuring at the 'top (highest point) of the withers' (TOTW), and for me, I would like to keep the height limits the same, and grandfather in any horse that were foaled prior to 2013 (or some other future date that would allow breeders to prepare for the change).
I don't believe that measuring at the TOTW would 'encourage' breeding for mutton withers. If it were true that the measuring location affected how people were breeding their horses, then I think you would be seeing people breed for manes that grew further down a horse's back and sacrificing 'quality' for the length of the mane. So, I think that saying measuring from the TOTW would result in people breeding for mutton withers is not a very good reason to reject using the TOTW as a measuring location.