She tries something, does well, and moves on to something else. This might just be what she needs to do in order to find out her true passion. I think 8 is a perfect age to dabble. Yes some kids might be really good at the first thing that they try, but it doesn't mean that this is the thing that they love the most or the only thing they are good at. I'd say, if you can afford, let her try different things for a while and see whether she gravitates toward certain areas. Sticking to something is good in that the kid will have focus and is likely to do well. But it also means less time to discover a possible "true" passion in something else.
My DS9 also has many interests and also excels at many things. Recently he started focusing on music and chess, so we let him decide what other activites he wants to drop and let him choose more chess and music activities. I think whether quitting is OK really depends on the reason behind it.
I also don't arrange for my two kids to do the same activities out of convenience, I think lots of parents do it but this would not help the younger kid develop his/her own identity. My two kids are fairly different in their interests so I try to give my younger one more room so that she will develop her own interests independently of her brother's. Easier said than done, though, the money and time that parents put in kids activities can get pretty scary.
