frtklau:
For the use of RX/TX to talk in a wifi shield, is 'speed' the only drawback ? (when compared to SPI, IIC for example).
I'm not sure that speed is much of a drawback. I would think that you could talk to it with speeds well in excess of 115,200, although there may be a software limitation in the Arduino library you might need to code around. Many Arduino's have just one UART, so if its talking to a shield, it can't be talking to your computer. SoftwareSerial, in my opinion, is not a very good way to do serial communications. Its a nice library and it works as well as possible, but serial communications like that really needs a UART.