These two things can be combined or separate. My idea is for a phone, even with the same interface as the app would be cool, but just the bar (to see the numbers pressed) and then a row of numbers for you to click (or a button to pop up contacts) and then a send button.
If the messaging widget is included then you still have the bar + number bar and then a message button next to the contacts button and dial button and it says message. You click that and a body input box appears with the full keyboard.
I really don't care about the messaging widget as much but I'd love for a phone widget, sometimes I just wanna quickly dial someone and don't wanna open phone and then click on phone (it's always in call log when you first click it) and then dial... especially useful in my "in car screen" (music widget, maps app, quick dial folder, places directory app).
It's just an idea I think others would use it too though.