It checks each wall of the hole it mines, so if you look carefully, you'll notice, that it would check every single block in the area, so it won't miss any ore.
And it is slower unless you combine this pattern with multi-turtle mining concept what is I want to do eventually.
Well, for every turtle you make, you can just as easily make another Quarry.
After you get to the point where you can make a quarry, it's no longer about how efficiently you can mine. It's about how fast you can take in resources.
Once you make your first quarry, it's just a matter of time to make the rest of them. I could easily have 5-10 quarries working for me at any given time if I expanded my boiler room. The reason I don't is that I simply don't have any need for that many materials. At the end of the day, quarries will always be faster than turtles. And when you have everything in terms of resources, the only thing that matters is time.