In a PFD, you need to feed the loop filter to keep the VCO stable. With an uncontrolled dead zone, you'll have a number of positive spikes, followed by a number of negative spikes, which causes jitter at the frequency of the loop filter response (=low frequency).
By making certain that the output of the PFD always outputs alternating polarity pulses at input frequency when locked, the jitter frequency is equal to half the input frequency, which means it much higher than the loop filter cutoff frequency and thus negligible.
Smart design, by the way.