Actually, there's one remaining advantage to iMessage and I was setting up to do it when I got involved in this thread but first, I digress... Originally, we both had it turned on and we had plenty of blue test messages. We also had blue test messages to one of our sons but we were completely unable to send any messages at all to our other son. At that time, iMessage was the furthest thing from my mind and I had no clue regarding the significance of blue messages. After much weeping and gnashing of teeth, I discovered that messages worked only to/from iPhones and that made me think about iMessage vs. SMS. Many years ago (with stupid phones), we had our carrier (Verizon) turn off messaging and totally forgot about it so we were unable to send SMS messages to our son with a non-Apple phone. I contacted Verizon and they turned messaging on for both of us and all was OK. Both of us left iMessaging off for no particular reason.
Then I learned that I can send iMessages to my grand-daughter with an iPod (and no phone number). She indicated that she trades messages with her friends with iPods and WiFi-only iPads. The darned 8-year old knows this stuff better than I do! I turned iMessaging on and planned to do a test with her the next time that I saw her. With the background of all that, I carelessly jumped into this thread.