Woosh!
I'm looking at the bigger picture for usability.
Irrespective of what you can use it with apart from that tablet, price, etc; at the end of the day, a cable, be that adaptor or pure cable: ie; an accessory, is needed to use them.
There's no woosh, I know what you're saying but if you're going to be pedantic about it you need an adaptor and a cable for the galaxy tab, as opposed to just a cable for the Xoom. Sure you may already have the cable to go with the adaptor, but then if you buy a mini-HDMI cable now for the Xoom you already have it next time you buy a device that uses it.