This is a common myth--you can add olive oil to butter and increase the temperature. Not at all true.
Butter burns because of the proteins in it. Adding olive oil does nothing to remove the proteins. If you want to cook something in butter at a higher heat, clarify the butter. But adding olive oil doesn't really do anything except change the flavor.
I personally enjoy mixing it up, so if I've added butter to the steak, I might cook the veggies in olive oil. Much nicer way to mix it up, in my opinion.
james peterson in his master piece "cooking" suggests using pure olive oil over extra virgin, which he says is expensive and loses its flavor when heated. there are plenty of instances where he recommends extra virgin, just not for frying or sauteeing.
i guess it depends on your income, but if you're destroying the flavor of the oil, why cook in it? if you want something flavorless, go with vegetable or canola oil. i use pure olive oil for most cooking, unless i'm making pizza dough, salad dressing, and maybe i'm forgetting a few things, but it's rare i reach for the expensive stuff.