Sure, but without cell service (as in a phone number instead of just data), aren't those just closed IM services? Don't get me wrong, I use Hangouts over SMS/M

Sure, but without cell service (as in a phone number instead of just data), aren't those just closed IM services? Don't get me wrong, I use Hangouts over SMS/MMS whenever possible but while I can reach anyone on Hangouts if they have it installed, I can't install iMessage since it only works on Apple devices. As such, I have to fall back to SMS to reach people on iPhones who haven't or won't install Hangouts. As much as I prefer IM over texting for the general features and speed, it's still useful as a fallback option if only because the various IM services aren't interoperable.