I never used to add oil to pasta, but my husband can only eat gluten free pasta, and to this it is an absolute must that you have to add olive oil to it, or it doesn't cook properly.
When I cook 'ordinary' pasta for myself, it has just become habit to add olive oil, and it doesn't seem to affect it badly.
Having said that, it does make the pasta slightly softer, but this doesn't matter to me, because that's how I like pasta anyway.
A good pasta dish, in my opinion really can't be beaten, and I could eat pasta any day of the week!