The problem with that approach is this: how do you know they have no ads until you buy them?
Unless the App Store introduces an ad-free indicator on the app!
I've often said that Apple should have three categories to avoid confusion:
Free
Ad Supported
Paid
Of course, a paid app shouldn't have ads. I'd request a refund if I ever bought an app and found ads in it.
I have a few apps that have the little banner ads in them, but any more than that and I won't use the app. Video ads? Not a chance. I despise the online flash players that have unskipable video ads, I'm not going to put up with them (skipable or not) on my iPhone.
I wonder if Apple will ever start demanding a piece of ad revenue. If too many apps go 'free' with the developers getting all the ad revenue, Apple will then be stuck distributing the apps for free, but not getting any profit.
I'm amazed that image conscious Apple has no problems with iPhones plastered with cheap ads. If anything cheapens the iPhone experiences, ads do.