While human error like you describe above certainly exists, these systems can also catch drug allergy interactions, drug-to-drug interactions, and even food-drug interactions. Along with the already-existing systems in most pharmacies, these systems provide another layer of protection for patients. They also provide doctors with real-time best-cost analyses, allowing them to prescribe the most effective, least expensive drugs based on a patient's particular drug coverage. This may help to lower the overall price of healthcare and insurance coverage.
Protection from the errors you describe isn't technologically insurmountable, either. Robotic systems that are linked to the prescription and automatically fill prescriptions eliminate the pharmacy errors, and EMRs that provide diagnosis/drug checking are likely right around the corner. Doctors don't like the latter much, however, because they are perceived as taking too much of the medical process out of their hands.