I have seen instances where people mix white sauce / bechamel with spaghetti or macaroni but never seen a reputable chef to do so. This mix is sometimes mistaken for carbonara since its creamy white. Is it authentic to do so? If not can please also expound what béchamel usually used for for added information.
In Italy, besciamella is usually only used on pasta when it's going to be cooked in the oven: pasta al gratin, pasta al forno, or as part of lasagne.
As for your question - is it authentic to just toss some pasta with it? I would use the same method I'd use to answer the infamous "is X a sandwich?" questions: if somebody asked for a plate of spaghetti at a restaurant and they came covered in besciamella, I think basically every Italian would be baffled. I've never seen it on a menu, never seen it on a table at anyone's place and never heard it discussed until I read this discussion, which makes me inclined to say it's not authentic. It could still be delicious though!
I use it to make a slightly less rich 'carbonara-esque' sauce with pasta; bacon [or pancetta] onion, garlic. Make a roux round that, then milk, mushrooms, cheese, done.
I always considered it just a poor-man's carbonara - though it's fabulously tasty.
However, though it seems to have fallen in popularity under its original name, salsa colla, Béchamel actually originated in Tuscany & was later taken to France & renamed.
I'd say that gives us every moral right to put it on pasta & call it 'authentic Italian'.