As a camera operator:
ND filters are nice especially for one reason: image sensors don't behave the same across the ISO range. In a very bright situation you could close the aperture or use a higher shutter speed, but many times this is not desired. If you had a perfect sensor, you could just turn down the brightness of the sensor – in reality however you could run in some kind of problems with this (for example colors look different, reduced dynamic range, different noise, ...) So sometimes the best solution is to use a ND filter.

You can always use a external ND filter (slap it in front of the lense), but having in-camera filters is way faster and safer (the filters are behind the lens, this way they are protected). If you are shooting in a production where the expensive thing is not the camera, but the time of 20+ people waiting for you to exchange that damned ND filter, a flip in ND can save you a lot of stress (which leads to better pictures).

So internal ND filters are by no means a must have for all scenarios, but if they are there they will be used.

I am not an expert on sensors, but I assume with the right sensor and a good implementation one could avoid having to use ND filters altogether. Note that

Another use for a special breed of ND filters is if you wanna pull the depth of field. It works like this: you have a variable ND filter and for example while you open the aperture you close the variable ND filter (this results into a change of depth of field without change in exposure).

Just use one of these: https://tiffen.com/about-138_vnd/

this electronic controlled ND filter built in would be the perfect solution for two reasons:

1. ND filter is important for documentary/ENG work
2. it can act as a shutter and mimic an mechanical shutter with the superb motion reproduction through soft fade in and out the time of exposure for each frame

http://www.lc-tec.se/products/variable-nd-filters/

In my opinion, I think an ND filter for the AXIOM is necessary. The option of the electronic ND filter integrated into the body of the camera is the best option :)

Agreed on the internal ND filter. Whether it's an electronic, variable, or rotary disk style, it's incredibly handy to have. Especially on cranes and gimbals. It could also breed greater functionality and cause for use of the remote control in those situations. Lack of behind the lens ND has made some RED rigs much larger than what would be preferable in some tighter scenarios as well.