"I would argue that tweaking WB in camera is much more time consuming than doing it in post."
It may be so using a bare canon SLR but definitely not so if ML provides the tool to do it.

I agree if things are happening randomly & u need to capture whatever pops up in your face,
u just shoot and forget about setting WB. However, for events and model shoots, u can afford
about 30sec's time to set correct WB and forget about that half an hours WB tweaking in post. I think that
is a good investment. Well, if u count the LR pic loading/exporting time and 1:1 rendering time, it saves much more time
than u think.
For my shoots, I usually can post my pics the same day as the shoot, because my jpgs are correct directly
out of the camera and I don't need to go into a raw converter for 95% of my shots.
Hv u tried WB shift bracketing in camera? U take 1 pic and only 1 pic and the camera would give u
3 jpgs with different WB to choose from. At the same time, I know ML can display "tweaked exposure" during LCD
playback using the thumb dial.
By combining the above ideas,
what would be handy for ML to do is that, during playback mode,
ML would give us 2 dials to tweak WB exactly the same way as the 2D WB-Shift. At the same time the RGB spot meter (in %)
would get updated upon the 2 dials. (one dial for Blue-Amber shift and another for Green-Magenta shift).
This way, skin tone WB can be set quickly: (u know all subsequent shots will hv good skin tone)
1) Take one pic of the person under that particular lighting with the best guestimate of WB/K setting and exposure.
2) Playback the portrait and activate ML Live 2D WB-shift. (I think K can be adjusted in liveview too, not sure about WB-shift)
3) Point the RGB % spot meter on the face
4) Rotate thumb dial and main dial to weak WB until u see a pleasing skin tone or until the RGB % spot meter gives u a good value.
5) Press a button to register that 2D-WB-shift into the camera.
6) Done!
Note it is not always possible to use a grey card, nor would grey card always give u the most pleasing skin tone.