Synchronous Communications (like I2C) is faster and more robust than Asynchronous Communications which in addition to wasteful Start and Stop Bits also requires stable clocks at each processor for fast data transfer. If you're already laying down an IC2 Bus for other devices, it seems logical to extend it to encompass your processors as well, and then your processors can happilly rely on internal oscillators freeing PIC pins. Remember that PICs internal oscillators are only guaranteed to be factory calibrated to 4MHz +/- 1% and regardlesswhat people say, that just doesn't cut it for Async speeds above 1200 Baud with any degree of reliability and repeatability... whereas I2C is happily interchanging data at between ten and forty times that speed without any regard to clock stability.