I use the DS400 all the time, on a daily basis, always painting houses and furniture and whatever that needs a colour. I also have the RS400, I use it less often but it comes in very handy for the places where the DS400 won't fit.
I used to have the RO90 and I like the sander, but I did not like the cost of its paper which is about 2,5 times as expensive as the paper for most of my other sanders, so it is not very cost effective. Add the fact that there was a lot of overlap from the RO90 with the 5 other sanders I had like the DS400, DX93 and RO150, and I made the decision to let it go.
But everybody does different things and has different needs. If the RO90 fits well in your workflow and the DS & RS400 don't, no need to keep 'em around. RS would be the first to go with me.