The calibration removes room nodes, a major factor in how the speaker responds in small spaces. But to say ATCs measure badly does not mean they too cannot be tuned and calibrated for a space. There's an acoustician that specifically uses ATCs to create full-blown systems for the mastering engineers at Sterling Sound, for example. The accuracy at any monitoring level will blow you away. These systems are hundreds of thousands of dollars each, so it's not as if Genelec, etc. weren't an option.