> the PICmicro has diodes of its own, so you might be able
> to get away with leaving out the diode

Any voltage on a pin below -0.6V leaves you open to latch-up,
which can destroy all or part of the PIC. If you want to clip the
negative portion of a 0V-centred AC voltage (or any portion
which could go below -0.6V or suppress below-ground noise
spikes for example) it is advisable to have a Schottky from pin
(a) to 0V (k). Also a 5V1 Zener (assuming 5V Vcc), also pin
(a), 0V (k) to prevent latch-up on the +ve side

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