Rubinia Soulsinger may not be the greatest Legendary Creature to build around in EDH. Still, people try. She's certainly pretty good in a deck that utilizes tap and untap shenanigans like the Evasive Maneuvers deck in which she was printed. Beyond being Mind Control on a stick, she's just a 2/3 for 2GWU, which is okay. But in the right deck, she's very solid and can cause a lot of issues when she's able to tap and untap at will.
True, there are cards in White, Blue, and even Green that pretty much do what she does. But, she's a classic Legendary creature, and some Derevi, Empyrial Tactician and Roon of the Hidden Realm EDH players have kept her in improved versions of the Evasive Manuevers deck. But she's hardly ever going to be popular as a Commander. Still, Bant is a great color combination to build a Commander deck around. So, as a relatively non-threatening Commander, you can build a sneaky good EDH deck around Rubinia Soulsinger.