Good point! Someone said to me that the in built colorimeter would check that the profile remained accurate, however, now you mention it, i agree it makes little sense. Whatever the case may be I'm pretty certain that the in built colorimeter is inferior.
When I bought my Eizo a few years back, there was a model that had a full on calibration device, and my model that has what is essentially just a white point adjustment device. The idea being that one would use an iOne Display Pro to perform the actual calibration, and one could set the internal device to keep the white point consistent between calibrations. And I found that the white point adjustment wasn't all that consistent anyways, so I don't use it and just recalibrate with the iOne Display Pro device.
I've also been told by an Eizo representative at a trade show, that with the newer displays, an iOne Display probe will, in general, be the better choice for calibration, but he didn't say why. It is possible though, that models that store a real 3D LUT in the display will be more accurate than displays that do not.
When I purchased my Eizo, I chose it over NEC because I needed it's frame rates for video work. For still photography this is irrelevant. I think now, the only advantage for the Eizo, are the models that let you calibrate with non-Eizo software and load the resulting 3D LUT into the display memory. This is far more useful for video workflows than still photo workflows. IOW, for still photography, the displays will probably be almost identical, so choose the less expensive. brand.