I agree with those that say its hard to teach/coach. I have this issue with one of my kids. She will kill her older brother when they play together, but on the field with other girls her age she's more "respectful". As I tell coaches (and parents) when I referee, soccer is a contact sport, and the best players don't fear contact, but more often thrive on it. They want the ball, they want engagement, they want to be the center of attention, they don't back down. That's what I refer to as aggressiveness.
As for tryouts, I agree as a coach that it is a mess. Retention and loyalty are important to teams and clubs, especially when players have siblings and friend within the club. "Sorry Mrs. Jones, your son Jimmy is just a tad subpar, but I hear coach John really wants your daughter Jane to stay with his team, as she's such a great player..." This makes it harder for coaches, when a team may need to be split/relegated due to talent coming in, or that needs to be jettisoned. Not fun stuff.
In the end, if development is paramount to winning, you'll keep some less talented players and not ask some betters on. There are always losers. If you have big club, this is easier, as there are 3-5 teams per age, and you can just move players around, collect their money and not tell them Rec is their better option.