MRP systems come in all shapes and sizes. Some are OK, some are great and some are astounding. Some have big price tags and some are modestly priced.
They all need a human input however...
I once made this remark to a client of mine and he said that he was going to write 'Human Intelligence Required!' on the whiteboard. I thought he meant his whiteboard in his office, but no, he meant the one in the Production Control department!
I kept quiet about the origins of the comment when I was next onsite, but non the less, the point was still valid.
MRP (and MRP II / ERP) is not a magic wand. It will not do all of the work for you. Sure it will do a great deal of the number crunching and 'donkey work', but there will be times when you need to think about its output and make a decision.
Blindly accepting the output is a desirable, but potentially damaging approach to take.
I've seen despatch programmes loaded for non-working days, schedules cutting across planned maintenance programmes and work to lists that don't make any sense.
If you are feeling frustrated with the results you are currently getting from your MRP system then it might be worth considering the phrase from earlier:
...optimising MRP systems and re-engineering business processes