I can see value in Rule Machine processing the SMS messages. Thinking of scenarios where you could SMS your Twilio number things like "den" to turn on the den lights, "garage" to open the garage door, etc. Since RM already has conditions and actions, it would nice to leverage that conditional logic to perform the actions based on the SMS. For example, if SMS received is "den" then action is turn on Den Lights. It would be a rather hard coded and complex app to build with all the scenarios versus just building rules. But I would expect it to be complex to add this functionality to RM as well.
Now all of this said, how would you manage security around this? I would hate for anyone, roboSMSer, etc to text your number and it perform some action. You would likely need some app sitting in between to verify like what is mentioned in this ST Forum article where the guy has a Node.js server that queries Google Contacts for verification before automating anything. This would be more difficult to build in Rule Machine. The only idea I have there is to query devices for details, but again complex.