Of course her powers are actually tremendously useful. It's just she never gets a mission that would take proper advantage of them. Imagine how great she'd be in a covert operation: she can be in two or three places at once! She could be having a conversation with the evil overlord, while at the same time rifling through his panty drawer. And eating pizza.
There are great stretches of Legion stories I haven't read yet, so it may be that someone did get around to writing her well, but there's not much sign of it in the two Showcase volumes so far available.
And it gets worse. In volume two, one of her selves is killed! One third of her being is destroyed! And so we get a touching little scene in which she says "Oh, guess I'll need a new name, then." You can almost feel the entire lack of emotion. By complete coincidence, the day I read the story, I also saw the episode of the Legion cartoon where T-Girl #3 gets offed, and they at least gave her a couple of minutes to be upset about it before changing her name to Duo Damsel.
And then to cap it all, in a recent storyline (mostly Countdown) she loses her second self, and renames herself Una. She now has the super power of being able to be one girl. I'm not sure if this technically counts as a super power anymore.