Why a professional is ordering the 5D MK III

If you consider seriously, D800 will provide better specs than 5D3, provided that you don't mind jumping ship.

No.0, D800 is $500 cheaper than 5D3

no. 1 reason for the 5D MK III: Dual card slots - D800 also has

no. 2 reason for the 5D MK III: Improved image quality - D800 with more pixel, downsized should at least match 5D3 quality (need proof in the future). Don't forget there is D800E

no. 3 reason for the 5D MK III: Enhanced dust removal - D800 is not bad too. But I don't think it is an important feature though

The 63 point metering system sounds swell. - D800 has 91000 pixels for metering. Don't forget it has spot metering for every focus point (not only the center one)

Just to list few based on your comments. Why don't choose D800 instead of 5D3?