The directionality of the tyres is linked to their ply construction! Ply layers are directional for added road stability. Force is carried by the plies lateral and diagonally to prevent sidewall compression on braking and on acceleration. Braking creates forces in opposite direction than does acceleration - one reason front tyres and rear tyres are different.
Water dispersion, contrary to popular belief, is NOT the main reason, because tyres do not act as squeegees. Instead, they press down on a water filmed road forcing the water into the tyre grooves. Any point in the rotation is as good as any other - there is no cascading sequence. i.e. Water is not squeezed along a groove to be released at the edges, and does not squirt out the sides as the tyre rotates, it but rather is carried by the tyre around and flung off tangentially to the rotation (Notice how tyre spray makes the wheel look like a pinwheel).
Reversing the direction puts extra stress on the sidewall when you brake and when you accelerate - shortening tyre life and increasing the chances of tyre decompression failure. Tyre wear is effected mostly, perhaps as much as halving tyre life. Chances of compression failure remains very very small even with the increased stress.
