And you're conflating two different definitions of security. We're not talking about remote root holes or buffer overflows when discussing phone security; we're talking about trojans and other maliciously designed apps which are downloaded and run voluntarily by the user. A walled garden does provide better security in this regard, as demonstrated by the iPhone having a perfect track record at almost 3 years in, and Android already having malware in it's App Store despite the platform still being in it's infancy.