We have had a mini bar and a low beam for about 2 years. Like a PP, DD's birthday/xmas list was always bar (for years), beam, mat, springboard, vault, etc. Didn't play with toys. That was her list. (It's now moved on to ipad, ipod, laptop, etc, but that's another story). I didn't cave on the springboard and vault though.
She practiced a lot of old level 4 skills on it for fun, really did help that mill circle. But I didn't get it for her to improve, I got it because it's a toy. No different than the monkey bars at playgrounds that the kids are always trying to flip on. The mini bars are too small to do anything crazy on, IMO. And i'd rather her play on that bar than the top of old swingsets like I used to.
She used it quite often for most of the 2 years (still uses the beam regularly). Now the 5 yr old uses it pretty much everyday. Good amount of use for the money, IMO. However, to answer the OP question, if she is the only kid who will use it, I don't think it's worth the money for a new level 4, not much can be done on it...