So you can have a part that isn't worth copying on a car that's good overall, but you can't have a part worth copying on a car that supposedly isn't very good because you've deemed it so?
Lets say you have TeamA, who innovate DeviceA.
Lets say that TeamA are finishing near the back.
The amount of resources which TeamB should spend attempting to copy TeamA/DeviceA should be less.
Why do I believe this?
Because if TeamA who are innovating the DeviceA are struggling, TeamB should not take the risk of possibly falling into the same trap that TeamA have fallen into. They are better of spending resources on another Device.
In the business World a similar philosophy is adopted. When a big company buys a smaller company, they value it. If the smaller company has produced a product/service that is not selling or performing as expected, they will value that company lower. However if that small company has produced a fantastic product that is selling like hot-cakes, the bigger company will have to commit a lot more resources to buy that smaller company.
In a nutshell, if a Team has produced a device and have proved that it is working great and delivering, teams should commit more resources into copying it. If however, the device is not proving itself, the Team should spend less resources on attempting to copy that device.
In actuality we have seen this happen. For example, in 2010 (I think it was), McLaren innovated the F-Duct. Once it was proved conclusively that the device worked, other teams copied it very quickly.
In 2009, BrawnGP produced their double decker diffuser. That moment it proved itself in battle (not before), teams started taking it seriously (Ferrari went to court) and soon after other teams developed their own.
Now, lets take a look at the failures. Williams produced the incredibly small back end in 2011. This design might've been great. But Williams did not prove that the system worked and had their worst season in their history of F1. How many teams copied this? I'll give you the answer. Zero.
In 2012, Ferrari have developed the pull-rod system. The device itself might be good, but Ferrari are having a disaster with it. How many people are going to attempt to copy it? I will tell you right now. Zero. If however, Ferrari had opened the season with the best car, I promise you that other teams would look into perhaps copying it (either for this season or next season).
What I'm saying is that in order to commit resources towards copying a system/device, it has to be proven in battle. Otherwise, other teams will do best to develop their own/other systems.
The moment that a device/system is proven in battle, its value/worth increases exponentially.
It would be great if every team could copy every single device being used by every other team. But resources are limited and a team must prioritise the distribution of their resources.