Champagne has a delicate taste. The taste and the bubbles will be cooked out. Champagne is expensive; why not use a cheap chardonnay - which is mostly what champagne is made from - instead?
I don’t get it. Is it just because having ‘champagne sauce’ on the menu sounds posh?
“Because it sounds posh” would be my first guess, yes. Then again, not all champagne is cheap chardonnay - you’re confusing champagne with “bubbly” 
I didn’t make that very clear. I know what champagne is made from: chardonnay, pinot noir, pinot meunier - either one or a mix. What I meant to say that as the Champagne region is so far north that the grapes don’t ripen enough to develop very exotic flavours and so isn’t able to retain its flavour in cooking, why not use a cheap chardonnay instead? Same result and less expenditure.
Flat champagne has a very different taste than Chardonnay. Most Chardonnays - especially the cheap ones - are pretty heavily oaked. Even if you get lucky and find an unoaked one (pretty rare in the lower price range), the method used to make champagne has more sugar and yeast in the mix.
The two are not the same thing, even if they start with the same grape.
I have actually drunk deliberately still chamapagne, and the one I had wasn’t very appealing. It has its own appellation: Coteaux Champenois.
I agree that the there is a difference in taste between flat champagne and chardonnay but the point I’m trying to make is that if you were to make one dish with champagne, and another with chardonnay (oaked or unoaked), they would taste the same. I haven’t tested this but I’m sure the cooking process would eliminate and disguise any difference.
Here’sa little blurb on cooking with Champagne:
Er, why would you think that?
Ingredients make a dish. Like any other ingredient, the quality and style of wine you use while cooking is going to influence the final dish. Beef burgundy made with crappy red wine doesn’t taste the same as beef burgundy made with a quality burgundy.
Champagne is most often used in delicate sauces or poaching. You’d definitely taste the difference if you used an oaked chardonnay instead of the champagne - it’s a very strong taste.
As I say, I haven’t ever had a taste test. You couldn’t quickly cook a couple of dishes and send them to me to try out?
But do the bubbles actually make a difference?
And yes, I can see that the oakiness might impart a richness, a weight to the dish. But does the *flavour *change? (Bearing in mind that the difference in flavours in the wines can be very subtle.)
The bubbles make no difference at all. Apart from the lack of oakiness that others have mentioned, champagne is also a lot more more acidic than regular wine. That makes a big difference in some dishes.
My stepfather told me once that if you don’t like the taste of your champagne when it’s gone flat, then you’re fooling yourself. Really good champagne should still be palatable when the fizz is gone provided you’ve not allowed oxygen to oxidize it.
That being said, the reason you cook with it is that it makes the best risotto of all time.