I think people use more of the mother sauces than they realize.
Thickening a roux with milk (béchamel) is a pretty common practice, to make macaroni and cheese, potatoes au gratin, creamed tuna, etc.
Most of us use tomato sauce frequently, though we may not make it from scratch. It wouldn't be out of line to suggest that ketchup is similar to a tomato sauce these days, though ketchup has an interesting lineage--it actually started out as a fish sauce, tomatoes weren't included until around 1800.
And as I wrote some months ago, using a roux to thicken a stock (velouté) is basically the same process as making gravy.
Hollandaise and mayonnaise are similar, both are an egg and oil suspension (and both can be easily broken.)
Brown sauce (espagnole) is probably the least commonly used mother sauce at home, and not all that common in commercial kitchens, though demi-glace concentrate shows up in many high end restaurants. Demi-glace is a secondary sauce that starts with Espagnole. I've made Sauce Robert a couple of times (demi-glace, mustard, onions and white wine), it's excellent with pork dishes.
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