I agree with myancey, tumble is not a broken skill but if you were to add it to your AC then yea it would be too useful and everyone that doesn't have a ridiculous AC due to medium/heavy armor would be using it. Not only this but if your just trying to get past a bad guy without the attack of opportunity, their are other ways around this (armband of elusive action MIC) Your dex is included in your AC for this reason. If you are a Dexterous character then you will be harder to hit due to the fact your AC includes dex.