I don't like them not allowing downgrades either but I can sort of understand why.Apple's refusal to do that (or allow any downgrading) is one of those things that lend credibility to the "planned obsolescence" argument.
On Android, security fixes are sent via Google Play store. On iOS, security fixes come with firmware updates. The same security holes that allow jailbreak could potentially be used by a third party with physical access to your device to install spyware and the like.
Let's also not forget iOS 6 has the Heartbleed SSL bug. I actually still have an iPad 4 running iOS 6. However, I use it for tasks that don't require security (web browsing, DRM-free epub reader, games, etc). No email, no banking, no shopping.