I think you understand the DX vs FX mode correctly. As you state it's only whether you crop when taking the image or later on in processing.
The only real advantages of doing a DX crop in camera are the reduced file size, the resulting greater buffer depth if doing a high speed sequence, and possibly frame rate.
The biggest disadvantage is that you never have the ability to change the crop. What if you wanted to take your perfectly framed DX crop and print it as an 8x10? You would have to crop some more off the long edge of the frame to fit the new aspect ratio. If you had content all the way to the edge that was important to you, the 8x10 wouldn't work. If instead you took the original in FX mode you could keep the full long dimension of the DX frame and add some area in the short dimension to fill the 8x10 space.
I'd rather use the extra storage to that I can crop to different aspect ratios in post processing if necessary or if change my mind downstream.
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