Why is the pasta often cooked in the sauce in al'arrabiata recipes?
While Italian cuisine is defined regionally, the majority of Italian pasta dishes have one cook the pasta until almost done, then finish in the condiment. While there are exceptions that include thick, long cooked sauces like Bolognese or sugos, finishing in the condiment serves several purposes: (a) the pasta and the condiment can be combined, (b) the pasta finishes cooking, (c) it absorbs the flavor of the condiment, and (d) the condiment (sauce) thickens and emulsifies from the addition of the starchy pasta cooking water (this can be controlled by adding cooking water and/or allowing it to cook off). Yes, it is more difficult than cooking separately, and it can be a little nuanced, but it is not very technical.  With a bit of practice, I think you will find this step worth it.