Good strikers are everywhere, world-class strikers are not. Even if he wasn't as crazy as he is he wouldn't make much of an impact in terms of football talent.
In every sport you have crazy individuals/sportsman but if they are really talented then they'll make it eventually, Mike Tyson is an example and tens of footballers and NBA players etc.
Mario blames his skin for being one of rare black player in Italy and that it's not easy, then you got someone like Tevez for instance who has lived in one of the most dangerous cities in the world, lost his bestfriend but would still make a difference because he IS talented.
Tevez is a great example. He was kinda crazy (obviously not Balotelli level of crazy) falling out with Ferguson and Mancini and was constantly whining about being homesick. We got him at the price that we did because of his instability. When he is focused on football he is world class when he is not he is very inconsistent. In his first two seasons at city he was on fire but then he went crazy and became terrible. The talent was always there and now we are reaping the rewards.
Marotta recognized his talent and given the small transfer fee and the modest salary he accepted to join us, he was a low risk high reward kind of player for us and it proved to be a great transfer. Had he been crazy with us too then it would have been an average signing as he didnt cost us much anyway.
I think the same applies to balotelli but on a different scale. His time in Milan under Allegri had 12 goals in 13 games. Shaarawy was carrying the Milan team in the first half of the season balotelli was on fire in the second half. He was also on fire at the start of his Inter career. I remember very well how often he scored great goals against Juve. He also had a good second season at City. The talent is there, the price is low and the potential reward is high. If it doesn't pay off then we are unharmed. We are not like liverpool and Milan where we really need him to be on form for the team not to collapse. The team is very well functioning and delivering results without him. We have very able strikers on board. He can only be a bonus both in terms of football and commercially.
I don't agree with you that those with talent always make it. Cassano is an example of talent than never flourished. Adriano was an unstoppable player and then mentally collapsed. Robinho was very talented but he never delivered consistently outside the brazillian national team. Balotelli and all of these players lack mental powers not technical ones.