Beale is a great attacking player. My problems with him is he has an erratic playing style. I don't mean in form, I mean in structure. I think as a 10 he would be far too detrimental to the attack as a whole because he struggles to stick to the set list (If you will allow me to use a music metaphor).
The reason why he can win or lose you a game, is not because of his own errors, but because when he does that at 14 blokes are going the other way so to speak.
He also lacks the passing game you'd want in a 10. He has a good short passing game and good instincts for passing but he has always struggled with long passing both ways.
Due to this I think he is a great attacking 12 in Super Rugby, but lacks the robustness in contact to go really well in test rugby in this position. I think it's required as you need to be able to crash it in attack when there's less opportunity and also less opportunity to hide in defence. Now don't get me wrong, I'm sure he could towel up the Irish or Italian there, but in a big game against a top 5 nation.
He's also a very good test 15 at his best. He is not great, because he is weak under the high ball. Izzy offers all Kurtley's strengths and none of his weaknesses, so at best Kurtley is the 2nd best fullback in Australia.
But I don't see him as a very good 10. He is at his best when he is able to rove and chime into attack when opportunities present themselves. I don't think he is better than average at managing a game and his erratic attacking behaviour makes it difficult to build attacks over phases.
That's my general thoughts anyway as to why he isn't a test starter for us.