Your instructor likely flies mostly in other people's airplanes, so what does he care about a tarp abrading the paint off when it flaps around in the wind. Bruce's covers are made from a material that is tough and UV protected on the outside and soft against the paint on the inside. The stitching, corners, straps, and grommet pockets are sewn to also not cause damage. Their covers last for years.
Everything Kendel said is accurate. A tarp will beat your plane to death.
There are cabin covers like Geoff described, ones that cover the cabin, wings and cowling and also full plane covers that Mike Marra has shown in another post.
These are all mission specific products that are the best options if you must park outside.
