It would cost more CPU cycles and would be slower. The reason is simple: You just get a random chunk of memory and everything what was before you in this chunk of memory is still there.

But I know of one exception that at least exist on iOS and Mac OS X: When you get a new memory page, it is nulled by the OS for security reasons. However, as long as this page is mapped into your application (or vice versa), this won't happen again.