maybe you don't get access to all the api functionality which programming in Obj C would allow, but it seems like a pretty good compromise.
if the app you want to create isn't that complicated, why should "creating" an app in titanium be any different to writing a native app?
There's some apps that just need to be written once and then ported, like games and stuff where the experience is self contained within the app and doesn't involve using the UI or the scheme of the platform.
Then, there's other apps that touch upon interface elements from the OS, interact more natively with the OS and are not self contained boxes. This kind of app will benefit from being developed specifically for the platform. You can draw on specific features from each platform and really make your app shine for your users.
If you're making a game, with its own interface and where everything stays within the game, hell yeah, use titanium or one of the many other solutions.
But if your app involves integration with the Ui or specific features of the platform, even other apps, it's best do everything from scratch.