The #1 reason those systems are moderately successful: When your sales person needs to write the logic for billing customer X, they'll try to make the customer contract more reasonable.

Normally sales people are deal-driven, and if promising the customer "14% off on the first three transactions on every rainy Tuesday, unless Monday was also rainy" gets the deal closed, then that's going in the contract.

No better way to ensure the contracts are reasonable, than making sure the person negotiating the deal feels the pain of billing it