Why do people only focus on 1 aspect (a negative one yes) of a 2-3 zone? Yes there will be some more uncontested passes, but certainly not all of them. If you give up say 10 extra shots uncontested, but on the other end you're able to defend the other 40 shots 50% better, then overall you have a better defense. On top of that you get more rebounds too.
I'm just pulling numbers out of my sleeve and I know they're not accurate, but it's to give an example that you need to look at the total picture. There hasn't been much research to accurately tell if the 2-3 zone is indeed a good option in certain matches, but I wouldn't dismiss it solely because there's a downside to it. It could very well be that the benefits outweigh the extra uncontested FG's.