The only reason to use a pillar/pendulum split in a mono deck is to help defend against earthquake/trident.
The only reason not to use a pillar/pendulum split is against steal, since you're giving them two choices instead of one.
Since it's fairly rare for steals to be wasted on pendulums, and earthquake is a fairly widely used card, I always use an even split when playing monos, and if using an odd number will use one more pillar than pendulum. (for the steal reason)
The one exception might be in a monoearth deck using protect artifact when expecting trident/earthquakes by the opponent, so that you could protect everything with one PA instead of two PAs for pillar/pend.