I doubt very much anyone would remove the tyres on a new or hired car to examine them for faults. Where as we really should. Many a hired vehicle has come my way with the most appalling tyres fitted. Partly due to cost, but mostly due to complete neglect.
On pick up I just jumped in and drive off. Loaded up and half way down the motorway to find the thing handling terribly.
I'd presume most people would show extreme caution with an unknown tyre or part worn.
While on the other hand, I believe German rules don't allow for an odd tyre in a set of 4. So if one is punctured or low on tread, all four are replaced. Story goes we get these shipped over as our rules are far more relaxed. Not sure how true that is, but to bin 3 perfectly useable tyres strikes me as an appalling waiste.
All very valid points so far it seems to me. It's equally daft to fit new tyres to badly set up car.
As usual I suppose the best thing is a bit of common sense.