Supergirl's youth and inexperience make for a lot of interesting adventures with her getting into troubles that Wonder Woman would know better than to get into.
So you want an annoying kid sidekick? I'm not against sidekicks, but an inexperienced sidekick getting into trouble in a movie about the grown-up heroes usually doesn't lead to a mini episode of Young Justice, it leads to something like this:
- "Shiny green rocks, they're so pretty ...argh! Help meee!"
- " 'Magical death trap' Hah, like that can harm me, Super... argh! Help meee!"
- "It's so dark in here and it's just the two of us, Martian Manhunter, let me light some torches ... hey, why are you screaming ... NO, don't run!"
- "Tim and Conner are totally gay for each other, but they won't admit it, I'll just borrow Diana's lasso for a minute and get them in a room together, teheheh. *5 minutes later* Okay, Tim wants to do Conner and can't stop himself undressing him with his eyes every time they're together and Conner is ... not interested at all, awwwkwaaaard"
Exactly, children have a way of getting into trouble that adults do not. Though shiny green rocks would defeat superman as well, I don't see Kara as any different from that. and I think Kryptonite overuse is a real problem, if the villain always pulls out his trust piece of kryptonite, what good is superman. Kryptonite is just an excuse for some pathetic human to beat up on superman and his cousin. There is a reason why Lex has a supersuit, because he doesn't always have that trusty piece of kryptonite handy. Wonder Woman would simply punch his lights out if he tried that green rock on her. On the other hand supergirl don't need bracelets to deflect bullets and tying her hands together would do nothing to stop her from breaking the rope and chain. I think supergirl is the opposite of being the fair maiden in need of rescuing all the time - that is Lois Lane's job.
More likely Supergirl goes about solving a problem the hard way rather than the easy way. Wonder Woman is probably better at avoiding conflicts than she is.