Pilot Doc said:
OK ... I'll bite.
Because it doesn't matter! If you specifically ask them to land on the centerline, and they can't - that's a problem. Lack of directional control causes accidents. If they land the airplane under control and 25' from the nearest runway edge, so what. Is a 30' wide runway unsafe? No. Neither is landing off center on a 75' wide runway.
How about being "only" 400 feet off your altitude? "Only" 5 miles off course? "Only" 30 gallons off on your fuel calculations?
Landing centerline gives you equal oppertunity to screw up either way. The only time you should be off centerline is when you DO screw up. And we ALL screw up from time to time. If you let yourself fall into the line of thinking that you describe, evenually you will find yourself in the bushes wondering what the heck happened.
If your instructor lets you get away with this without trying to correct it, find a new instructor!
For a supposedly professional pilot such as a Flight Instructor, consistantly off centerline is not acceptable. For such a person who aspires to fly large aircraft one day, it is really not acceptable. The larger the bird, the less screw up room you have. On something like a 747 or other large aircraft even when you are exactly on center line your main gear is only a few feet from the weeds and your wings are 50 or more feet out over the weeds.
Same holds true for taxi lines, get used to not following them and sooner or later you will take out a wing tip on a building, vehicle, airport sign etc.
Do yourself a favor doc, hold yourself to a little higher standard. I hope you do not operate with the same mentality. "Well I only cut 6 inches away from where I needed to, but who needs a spleen anyhow right?"
Off while learning is one thing, off as a low time PPL from time to time, acceptable as long as it's not the norm, off as a Commercial pilot or better and not caring? You have no business as a Comm pilot.