The "parcel of air" theory is all good and well until that parcel of air gusts, wanes, rises, sinks, sheers, or otherwise changes direction and speed - usually close to the ground. Gravity is a constant. Wind is not. That parcel of air you are moving in can also move around you. A 10kt gust on tailwind loses 10kt over the wing - instantly, and for a brief moment, due to inertia. Your ASI doesn't lie.
The reason for caution turning upwind or downwind (even if that happens to be crosswind) is because that's the direction the wind is likely to gust or drop off - leaving you in the poo if you are low and slow and in a turn losing lift.
We all know this, so I'm not sure why the argument.