Yes, I did sell my 300mm and 500mm and purchased the 400mm DO, and yes it does make sense, at least for me.
The 300mm was going unused, and I was finding the 500mm was receiving infrequent use because of its size and weight. I ended up using the 100-400mm zoom instead, even though its quality isn't close to that of the others. As is always the case, it's better to get the shot with an inferior lens than to not get the shot at all.
The 400DO which I first tested it in 2002, when it came out, was very exciting, but optically left a bit to be desired. Since then production lenses have been greatly improved and I can find nothing to fault. I know numerous photographers using the 400DO and don't know anyone with anything negative to say, other than that occasionally specular highlights can have a "different" look.
No, the MTF charts of the DO are not as good as this of the other Canon super telephotos, which are as good as it gets. But, in the real world image quality is simply excellent. I have recently done some shooting with the lens and a 1.4X, and even a 2X, and find the results to be perfectly acceptable, something that I don't often see with lesser lenses.
If practicality of use trumps absolute theoretical charts for you, then I highly recommend the 400mm DO.
Michael