Because the purpose for MessageManager.currentNum was keeping the key value for a lot of operations. As I told you before this patch only want to change from 'phonenumber' to 'threadid', keeping the rest of the code. Once we have 'thread id' stuff in Gecko we could change following your suggestions (I agree with the majority of them), but not in this 'surgical' change for restoring SMS App in v1.0.1