In general, the internal oscillator IS accurate enough for serial in most cases. But YOU need to calculate the possible maximum possible error between tx and rx. Read datasheet. Also note that the UART baud rate can only be "tuned" to digitally quantified numbers in the baud field. Sometimes we find that baud field has to be 24.3333333.... Well, I can't enter that. I have to enter 24, in doing so I create a +1.4% baud rate mismatch from the intended rate, ok in itself, but it contributes to the total error.
If the FT232RI has that 12MHz osc available on a pin, why not use that as the PIC's external OSC input? Since it's used for USB, it's gotta be more accurate than the PIC's internal OSC. But the brilliance is that error won't matter. Say the FT232RI was crazy messed up and the clock was 10% too fast. OK, the USB might not work, but the point I want to make is that it'll send and read the serial data 10% too fast too, but... the PIC will be clocked 10% too fast so it sends and receives 10% too fast, exactly in sync with the FT232.
Curiously enough, IIRC the UART baud rate doesn't matter, only a certain % of baud mismatch error is tolerable before the last bit of a serial byte falls outside the margin of the majority-detection sample period.