I don't think that user interfaces should evolve in they way they look like, but they should evolve in the way they actually work, they way information is structured and presented to the user. It is the feel that has to change, not (so much) the look.
When I talk about interface guidelines I do not only mean the color and shape of buttons, when to use and when to not use animations or the margin and padding of elements - but much more how we've been trained to actually structure our applications and the workflow. We have learned to use (both when creating and consuming applications) a very narrow set of widgets and workflows which became an artifical limitation to the usability. The video I've linked is quite a good example.
Our world is full of non-standardized, task-centric, but yet very easy to consume user interfaces. Think of vending or ticket machines, cash dispenser or professional group-specfic interfaces, like medical devices, design software or industrial machinery. Most applications are created to cater a specific task and they deserve a specific interface. As you correctly said, one size doesn't fit all.
I think the visual appearance and the appeal of applications is another important, yet neglected aspect. Humans are visual creatures, whose brain is trained to perceive and associate information. That's the reason we still use whiteboards, mindmaps and paper to be creative, because the typical application is just flat. It presents us a bunch of information with no visual connection. We tend to group information types together (actions at the top, status at the bottom, additional infromation in dialogs), instead of the information itself.
I also think that the "learning effect" argument is stressed way to much, for the simple fact that even a standard-conform application has to learned, a non-standard conform application is not automatically harder to learn (quite contrary to, when done correctly it will be much easier to learn) and that it is always better to have an application which is hard to learn and efficient to use instead of beeing easy to learn and inefficient to use.
Yes, there is a huge potential to non-standard designs to be abused and good user interface design actually is rocket science, but there is also a huge potential in improved usability. Saying that every application has to conform to the standard and doing not so is actually an anti-pattern (and thus results in less valuable applications) is flat out wrong.
