Besides agreeing with Durangojo, I wonder if there isn't a basic misunderstanding going on here. You wrote,
To me it doesn't make sense to cook the pasta in the sauce.
We may be on wildly different pages. You seem to be thinking we actually cook the pasta entirely in the sauce. No, no. We're not quite that pazzo.
We're talking about a technique in which the pasta is cooked in water, removed and drained when it's barely al dente, added to a skillet on the flame with some sauce in it, where both are tossed together. The sauced pasta is then turned out, plated and served. That's the generic, Italian way to handle most pasta and sauce combinations.
Some are are so delicate to require special handling. For a few things you want to stay entirely off the flame and rely on the residual heat from the pasta, plus a little pasta water. They're areall the heat necessary to cook an egg sauce like a traditional Carbonera or melt an al burro like "real" Alfredo* to velvet. Anymore would curdle one and break the other.
But, by and large, Plan A is a brief heat on the flame, tossing the pasta and sauce. For a home cook, it can be difficult to explain to one's spouse why you need to get another pan dirty if you're only going to use it for 30 seconds, her (or his) mom doesn't do it, if you like Mario Batali so much why don't you marry him, etc., etc. However in a restaurant environment it should be SOP and a non-issue.
* We probably mean different things by Alfredo. The version I'm talking about is butter and cheese only -- creamed to emulsification. It doesn't call for additional cream and doesn't get any thickening whether from egg, flour or reduction.
Edited by boar_d_laze - 10/3/10 at 10:40am