Any "career" operator would beg to differ on preferring a monitor for the reasons already mentioned.
Yes there are times when an external monitor is better, but from a craft and movement point of view, I don't know any professional operators that prefer to work off of monitors unless they have to.
When you look through an evf it's about how connect physically to the camera, how you interface with it, how you move it. It's the same with many elements like the hand grips, the shoulder pad, the position of the handles, the material the handles are made from. Your whole body is more connected to the camera when you're looking through the EVF.
https://flic.kr/p/pFz1GqHaving both eyes open
https://flic.kr/p/8NFkYdOperating from monitors tends to happen more with the "out in front" dslr style shooters who are used to having a camera held in front of their chest.
https://flic.kr/p/fa8bZ6https://flic.kr/p/pXNnwn (using a cine saddle for stability)
https://flic.kr/p/hbu7Mi (when it's light enough)
Having a monitor in a position that is viewable when the camera is on your shoulder usually means having the to put the camera in a very awkward position in terms of balance fore and aft. In my experience, so that you're not testing your close focus capabilities, that means the monitor ends up out in front of you and it's often in front of the matteboxe, depending on the lens being used.
A lot of RED users also started shooting this way, simply because the early RED EVF's weren't very reliable and were very late shipping. Same again with the EPIC, they EVF was delayed like nearly 2 years.
https://flic.kr/p/6eG76vRelying here on an after market EPL. When operating a geared head like this the rod keeps the EVF in the same place no matter what the camera is doing. You'd have to take the monitor off board for that to happen on a geared head.
Having to shoot EPIC without an EVF
https://flic.kr/p/pXN6A4Operating a camera is a very personal choice about how you dress and balance your camera.
https://flic.kr/p/p2bQamAs I mentioned, most operators that make a career from being operators, prefer to use an EVF, but there's no rules to say you can't make a go of it if that's how you prefer to work.
JB