People that do this normally come from a position of ignorance. Having extra pixels doesn't cost you anything these days (early sensors had a small costs in high ISO, but no longer), it only comes with advantages in detail and flexibility, even if down sampled. Arguments about memory cards and storage are laughable really- between the time the 12MP D700 was released and the D800 was released average memory and disk prices had decreased a greater value per GB. Were people complaining at the 12MP cameras that were relative much more expensive on storage?
It is a wholly unsustainable position, the next iteration of cameras will all be in the 40-50MP range for the high resolution bodies, 30MP+ will be the new low resolution body. 1D/D5 bodies might be 24mp, but affordable prosumer bodies will be much higher.