A fair point but as mentioned earlier would allowing a fare look up on ticket machines not entirely solve this issue very simply?
Showing the single fares and caps side by side makes it a lot easier, especially if you're planning a return trip.
At the moment you have to look up the single fare, then run a new query for the return trip (as the fare and peak restrictions won't necessarily be the same - direction-sensitive peak restrictions are further complexity introduced in 2011) and then you have to look up the daily cap separately, as it's sometimes less than the sum of the two singles.
Unless you use Mike's excellent fare finder, of course, but I believe PAYG needs be judged according to the information readily available from the official sources.
Very often the operator of the train(s) you intend to use (which is a perfectly reasonable place for passengers to expect to find fares information) doesn't tell you what the Oyster or Contactless PAYG fare is. The best you can hope for is that their website will direct you to the TfL fares page, which is okay but hardly seamless. If the fares were the same everywhere within the zones, every TOC website could just have a copy of the fare chart on it, which could be used either in combination with their map of their network showing their stations within the London zones, or with the more comprehensive rail and tube services map.