Well, you only have one byte of receive buffering provided by the 8051. So, if after dispatching your subroutine you do not return to your loop in time to catch the next byte, you lose it. That can be a problem. Interrupt-driven serial I/O can help you out by making sure you always grab incoming bytes as they become available in SBUF. In all but the most trivial applications, this is usually necessary, along with a software FIFO/UART in which to store those bytes. In addition, for applications where the microcontroller must perform significant processing on the data received, it may be necessary to implement flow control or some kind of request-response protocol in your application to prevent overflow of the buffer.

--Sasha Jevtic