Dailies dictate how much you can do in a day. Timeless Isle-styled content doesn't.
It's important that the Timeless Isle is just an example of the model, not the be-all end-all. Future content in that style (like a lot of Warlords of Draenor's max-level outdoor content) is likely to be tweaked to accommodate feedback. At the end of the day, if you can provide the same kind of content that dailies have in an environment free of the actual daily restriction, then there's no real reason to go with a restrictive daily model.