The SIMs usually have a phone number associated with them and I've wondered if the Cellular iPads could do SMS texting using that number (let's leave iMessage and SMS forwarding from iPhones out of the picture).
Everything I found says NO, and sending a text to the iPad's phone number doesn't make anything happen.
BUT... I just flew cross country with my iPad 2 with a free T-Mobile 200MB/mo SIM from years ago still active in it, and I didn't put it in Airplane mode before putting it in my briefcase.
When I landed there was a new TEXT messages notification on the iPad from T-Mobile saying "Welcome to Canada" and telling me the rates.
So somehow T-Mobile is able to send a text to a Cellular iPad but it seems the general public can't.