For the use of RX/TX to talk in a wifi shield, is 'speed' the only drawback ? (when compared to SPI, IIC for example).
Your nice, expensive WiFi shield (Hydrogen = $75 !) may end up collecting dust. Mine does :-/
There is no official Arduino WiFi library, because there is no official Arduino WiFi Shield. Obviously a WiFi library needs to support the WiFi chip /module that is used that particular WiFi Shield.