Years ago when I was shooting with an Olympus OM4T I got used to using its spot metering system by which I could spot-meter on on up to 8 points in a scene. Then the camera would store and average the readings for the shot. With this very sophisticated system I learned how to effectively use spot metering to produce some amazing transparency images.
When I switched from Olympus to Nikon, I did so because Nikon had a spot metering system while Canon did not. When I added medium format to my photographic repertoire, I purchased a 1-degree Sekonic light meter so I could do zone system photography. Now I rarely use my Pentax and Rollei MF cameras.
So far I have not used my hand-held spot meter with my D800. But when I am doing more landscape work this spring, I may well get the old Sekonic out of the mothballs and see what I think of using it with the D800. I find that I shoot and think differently when I am using a hand-held meter than when I am not. Which is to say, I think more like a MF photographer than a 35mm photographer, taking far few photos and shooting very deliberately.
I frankly doubt that my exposures will be different between hand-held and in-camera metering. But I know that using a hand-held meter will slow down my shooting process and, hopefully, help me take the care I take when I am shooting medium format.