It really all depends on what you are looking for. There is an analogy with going on holiday. If all you are going to do is get up every morning at seven o'clock, go touring and not return until late at night, you hardly need a hotel with a very good restaurant. You won't be eating there. When you are negotiating with your agency you might reflect upon one simple fact.
Many clients do.
For this reason, you should not begrudge your agency a decent profit. It seems a wonderful idea, and in theory it is. But in practice very few clients are both willing to work on a results basis and also willing to allow the agency to control all the factors that would govern those results. In the advertising field, I note that clients are increasingly looking at this sort of approach. Some large companies have recently started proposing bonuses to their agencies in a reaction against the world-wide trend to cutting agency income.