Reference: http://venturebeat.com/2013/03/07/f-secure-android-malware-r...
Developers are generally intelligent people but unfortunately many users are not. Sandboxing is for those people and it does its job well.
I do think however that there is a case for allowing a vendor-level sandbox so that apps made by the same vendor could share data amongst themselves.
This would facilitate free-trial to paid app upgrades.
In any case, give it time. Apple tends to start off with a solid base and build from there (rather than throwing everything onto the wall to see what sticks).