i don't get the objection. from an engineering standpoint, the keurig machine seems to be a way to mix hot water with the contents of a packet to make a single cup of hot drink. it should be relatively easy to see to it that the flavours of the various packets don't contaminate each other, shouldn't it?
It's not perfect. The bottom and top of the cup are punctured, and the forced water tends to splatter a bit inside of the cup receptacle. Mine always has a few grounds, and the surfaces all have some residual coffee. The water doesn't run directly out of the bottom of the cup where it's been punctured, otherwise, you'd get grounds in your drink. Instead, it runs along a channel down into your cup, another place for residual flavor. Of course I clean it from time to time, but I don't do a ton of different flavors. The cleaning to get chicken broth or tomato soup out would be too much to switch back and forth.
At my office they have fruity tea options and I can tell you that the flavour gets into the coffee if someone previously brewed such teas. I don't want to imagine what a coffee brewed after one of this soups would taste like.