That's where I get lost. I've sent plenty of texts, pics, and video to non iPhone (mainly Android) users and they get them just fine. The only issue is the emoticons from my iPhone don't translate over the same.
It's the ability to group message with non ios users. iMessages is terrific for group messaging, but group sms messaging on ios is no good.
Also, if you IMessage someone overseas and they have their devices off, it will send as an expensive international sms. All viber messages are free.
I've never looked at stickers.