I will never, ever, understand the "online" status feature. It is ridiculously invasive and cannot ever be turned off or even hidden from specific contacts. This is a SMS client, not a chat client, and revealing a user's online status is highly problematic both socially and security-wise. I can't help but wonder how many lives this feature has ruined.
(I am talking specifically about the "online" indication, not the "last seen" information, which is also invasive but can be turned off).