The great advantage is of course the extra battery and that extra power means I can shoot for longer and for me the two batteries probably mean I could shoot all day with the camera.
Some will find it too heavy and bulky, but it's still small and light for a 35mm / 'Full Frame' sensor camera and it is of course very useful for any kind of 'professional' shooting. And since the A7r is obviously going to be used for that, it's an important consideration.
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